check-env: don't execute check-meta.nix 15,000 times

Generated from https://github.com/NixOS/nix/pull/2761:

```
                                                                                          ns     calls ns/call
- /home/grahamc/projects/github.com/NixOS/nixpkgs/pkgs/stdenv/generic/check-meta.nix:22:5 591200 15026 39.3451
+ /home/grahamc/projects/github.com/NixOS/nixpkgs/pkgs/stdenv/generic/check-meta.nix:22:5 8744   308   28.3896
```

more, generated by:

```
$ NIX_SHOW_STATS=1 NIX_COUNT_CALLS=1 nix-instantiate ./pkgs/top-level/release.nix -A unstable > before 2>&1
$ jq -r '.functions | map((.name + ":" + .file + ":" + (.line|tostring) + ":" + (.column|tostring) + " " + (.count|tostring))) | .[]' before | sort  > before.list
```

applying this patch, then:

```
$ NIX_SHOW_STATS=1 NIX_COUNT_CALLS=1 nix-instantiate ./pkgs/top-level/release.nix -A unstable > after 2>&1
$ jq -r '.functions | map((.name + ":" + .file + ":" + (.line|tostring) + ":" + (.column|tostring) + " " + (.count|tostring))) | .[]' after | sort  > after.list
```

and then diffing before.list and after.list to get:

```
                                                                                                        calls
- :/home/grahamc/projects/github.com/NixOS/nixpkgs/pkgs/stdenv/generic/check-meta.nix:4:1               7513
+ :/home/grahamc/projects/github.com/NixOS/nixpkgs/pkgs/stdenv/generic/check-meta.nix:4:1               154

- mutuallyExclusive:/home/grahamc/projects/github.com/NixOS/nixpkgs/lib/lists.nix:658:23                7513
+ mutuallyExclusive:/home/grahamc/projects/github.com/NixOS/nixpkgs/lib/lists.nix:658:23                154

- mutuallyExclusive:/home/grahamc/projects/github.com/NixOS/nixpkgs/lib/lists.nix:658:26                7513
+ mutuallyExclusive:/home/grahamc/projects/github.com/NixOS/nixpkgs/lib/lists.nix:658:26                154

- onlyLicenses:/home/grahamc/projects/github.com/NixOS/nixpkgs/pkgs/stdenv/generic/check-meta.nix:21:18 15026
+ onlyLicenses:/home/grahamc/projects/github.com/NixOS/nixpkgs/pkgs/stdenv/generic/check-meta.nix:21:18 308
```

The following information is from `NIX_SHOW_STATS=1 GC_INITIAL_HEAP_SIZE=4g nix-env -f ./outpaths.nix -qaP --no-name --out-path --arg checkMeta true`:

| stat                       | before         | after          | Δ               | Δ%      |
|:---------------------------|---------------:|---------------:|:----------------|--------:|
| **cpuTime**                |        179.915 |        145.543 | 🡖 34.372        | -19.10% |
| **envs-bytes**             |  3,900,878,824 |  3,599,483,208 | 🡖 301,395,616   |  -7.73% |
| **envs-elements**          |    214,426,071 |    185,881,709 | 🡖 28,544,362    | -13.31% |
| **envs-number**            |    136,591,891 |    132,026,846 | 🡖 4,565,045     |  -3.34% |
| **gc-heapSize**            | 11,400,048,640 | 12,314,890,240 | 🡕 914,841,600   |   8.02% |
| **gc-totalBytes**          | 25,976,902,560 | 24,510,740,176 | 🡖 1,466,162,384 |  -5.64% |
| **list-bytes**             |  1,665,290,080 |  1,665,290,080 | 0               |         |
| **list-concats**           |      7,264,417 |      7,264,417 | 0               |         |
| **list-elements**          |    208,161,260 |    208,161,260 | 0               |         |
| **nrAvoided**              |    191,359,386 |    179,693,661 | 🡖 11,665,725    |  -6.10% |
| **nrFunctionCalls**        |    119,665,062 |    116,348,547 | 🡖 3,316,515     |  -2.77% |
| **nrLookups**              |     80,996,257 |     76,069,825 | 🡖 4,926,432     |  -6.08% |
| **nrOpUpdateValuesCopied** |    213,930,649 |    213,930,649 | 0               |         |
| **nrOpUpdates**            |     12,025,937 |     12,025,937 | 0               |         |
| **nrPrimOpCalls**          |     88,105,604 |     86,451,598 | 🡖 1,654,006     |  -1.88% |
| **nrThunks**               |    196,842,044 |    175,126,701 | 🡖 21,715,343    | -11.03% |
| **sets-bytes**             |  7,678,425,776 |  7,285,767,928 | 🡖 392,657,848   |  -5.11% |
| **sets-elements**          |    310,241,340 |    294,373,227 | 🡖 15,868,113    |  -5.11% |
| **sets-number**            |     29,079,202 |     27,601,310 | 🡖 1,477,892     |  -5.08% |
| **sizes-Attr**             |             24 |             24 | 0               |         |
| **sizes-Bindings**         |              8 |              8 | 0               |         |
| **sizes-Env**              |             16 |             16 | 0               |         |
| **sizes-Value**            |             24 |             24 | 0               |         |
| **symbols-bytes**          |     16,474,666 |     16,474,676 | 🡕 10            |   0.00% |
| **symbols-number**         |        376,426 |        376,427 | 🡕 1             |   0.00% |
| **values-bytes**           |  6,856,506,288 |  6,316,585,560 | 🡖 539,920,728   |  -7.87% |
| **values-number**          |    285,687,762 |    263,191,065 | 🡖 22,496,697    |  -7.87% |

The following information is from `NIX_SHOW_STATS=1 GC_INITIAL_HEAP_SIZE=4g nix-instantiate ./nixos/release-combined.nix -A tested`:

| stat                       | before         | after          | Δ               | Δ%     |
|:---------------------------|---------------:|---------------:|:----------------|-------:|
| **cpuTime**                |        256.071 |        237.531 | 🡖 18.54         | -7.24% |
| **envs-bytes**             |  7,111,004,192 |  7,041,478,520 | 🡖 69,525,672    | -0.98% |
| **envs-elements**          |    346,236,940 |    339,588,487 | 🡖 6,648,453     | -1.92% |
| **envs-number**            |    271,319,292 |    270,298,164 | 🡖 1,021,128     | -0.38% |
| **gc-heapSize**            |  8,995,291,136 | 10,110,009,344 | 🡕 1,114,718,208 | 12.39% |
| **gc-totalBytes**          | 37,172,737,408 | 36,878,391,888 | 🡖 294,345,520   | -0.79% |
| **list-bytes**             |  1,886,162,656 |  1,886,163,472 | 🡕 816           |  0.00% |
| **list-concats**           |      6,898,114 |      6,898,114 | 0               |        |
| **list-elements**          |    235,770,332 |    235,770,434 | 🡕 102           |  0.00% |
| **nrAvoided**              |    328,829,821 |    326,618,157 | 🡖 2,211,664     | -0.67% |
| **nrFunctionCalls**        |    240,850,845 |    239,998,495 | 🡖 852,350       | -0.35% |
| **nrLookups**              |    144,849,632 |    142,126,339 | 🡖 2,723,293     | -1.88% |
| **nrOpUpdateValuesCopied** |    251,032,504 |    251,032,504 | 0               |        |
| **nrOpUpdates**            |     17,903,110 |     17,903,110 | 0               |        |
| **nrPrimOpCalls**          |    140,674,913 |    139,485,975 | 🡖 1,188,938     | -0.85% |
| **nrThunks**               |    294,643,131 |    288,678,022 | 🡖 5,965,109     | -2.02% |
| **sets-bytes**             |  9,464,322,192 |  9,456,172,048 | 🡖 8,150,144     | -0.09% |
| **sets-elements**          |    377,474,889 |    377,134,877 | 🡖 340,012       | -0.09% |
| **sets-number**            |     50,615,607 |     50,616,875 | 🡕 1,268         |  0.00% |
| **sizes-Attr**             |             24 |             24 | 0               |        |
| **sizes-Bindings**         |              8 |              8 | 0               |        |
| **sizes-Env**              |             16 |             16 | 0               |        |
| **sizes-Value**            |             24 |             24 | 0               |        |
| **symbols-bytes**          |      3,147,102 |      3,147,064 | 🡖 38            | -0.00% |
| **symbols-number**         |         82,819 |         82,819 | 0               |        |
| **values-bytes**           | 11,147,448,768 | 10,996,111,512 | 🡖 151,337,256   | -1.36% |
| **values-number**          |    464,477,032 |    458,171,313 | 🡖 6,305,719     | -1.36% |