nix: use boehmgc with enableLargeConfig = true

Fixes #43015 for me and hopefully also similar issues.

== Resource consumption ==

TL;DR: no change for small-memory cases, less CPU for large-memory cases.

I assume almost all of the large memory usage is just the expression
evaluation and managed by the GC, so I used just `nix-env -q...` to test.
Old and new lines for each command follow.  I tried to run each several
times, but the values were very stable (<1% difference on re-runs),
so only one line for each command-version pair is provided.

$ time nix-env -f . -qaP --description -A nix >/dev/null
- 0.06user 0.01system 0:00.07elapsed 101%CPU (0avgtext+0avgdata 29036maxresident)k
+ 0.06user 0.01system 0:00.07elapsed 102%CPU (0avgtext+0avgdata 29864maxresident)k

$ time nix-env -f . -qaP --description >/dev/null
- 6.45user 0.36system 0:06.82elapsed  99%CPU (0avgtext+0avgdata 1021024maxresident)k
+ 6.23user 0.33system 0:06.57elapsed 100%CPU (0avgtext+0avgdata  938408maxresident)k

$ time nix-env -f . --show-trace -qa --drv-path --system --meta --xml 2>&1 >/dev/null
- 56.35user 0.96system 0:31.03elapsed 184%CPU (0avgtext+0avgdata 3207708maxresident)k
+ 44.80user 0.91system 0:26.12elapsed 175%CPU (0avgtext+0avgdata 3192696maxresident)k

$ time ./result-nix-large/bin/nix-instantiate --dry-run --eval --strict \
    --show-trace ./maintainers/scripts/eval-release.nix > /dev/null
- Too many heap sections: Increase MAXHINCR or MAX_HEAP_SECTS
- Command terminated by signal 6
- 175.18user 2.68system 1:17.42elapsed 229%CPU (0avgtext+0avgdata 8468440maxresident)k
+ 178.48user 2.78system 1:15.11elapsed 241%CPU (0avgtext+0avgdata 8460572maxresident)k