nixos/release-combined.nix: makes aarch64-linux limited support

This is because it will not eval properly with `hydra-eval-jobs`.

```
$ ...hydra/result/bin/hydra-eval-jobs \
    --arg nixpkgs '{ outPath = ./.; revCount = 123; shortRev = "4567"; }' \
    -I "$PWD" \
    nixos/release-combined.nix
```

It fails with:

```
Too many heap sections: Increase MAXHINCR or MAX_HEAP_SECTS
```