macOS support for NixOS tests (#282401)

Closes #193336
Closes #261694
Related to #108984

The goal here was to get the following flake to build and run on
`aarch64-darwin`:

```nix
{ inputs.nixpkgs.url = <this branch>;

  outputs = { nixpkgs, ... }: {
    checks.aarch64-darwin.default =
      nixpkgs.legacyPackages.aarch64-darwin.nixosTest {
        name = "test";

        nodes.machine = { };

        testScript = "";
      };
  };
}
```

… and after this change it does.  There's no longer a need for the
user to set `nodes.*.nixpkgs.pkgs` or
`nodes.*.virtualisation.host.pkgs` as the correct values are inferred
from the host system.

0 files changed, 0 insertions, 0 deletions