workflows/eval: Ensure NixOS modules meta is valid

As observed in #484155, it was possible for broken meta information to
slip its way into the NixOS modules system. It looks like that data was
never checked. At this point I wonder if it ever was *used* in a
programmatic manner, given how long it took for it to get noticed.

This simple check causes the results to be evaluated in a format that
isn't "Nix-brained". Not using `--json` *could* allow `<LAMBDA>` to be
in the output, which is AFAICT undesirable.

```
 $ nix-instantiate --strict --eval --expr '{ x = a: a; }'
{ x = <LAMBDA>; }

 $ nix-instantiate --strict --eval --expr --json '{ x = a: a; }'
error:
       … while evaluating attribute 'x'
         at «string»:1:3:
            1| { x = a: a; }
             |   ^

       error: cannot convert a function to JSON
       at «string»:1:3:
            1| { x = a: a; }
             |   ^
```

.github/workflows/eval.yml

@@ -459,3 +459,7 @@ jobs:
       - name: Query nixpkgs with aliases enabled to check for basic syntax errors
         run: |
           time nix-env -I ./nixpkgs/untrusted -f ./nixpkgs/untrusted -qa '*' --option restrict-eval true --option allow-import-from-derivation false >/dev/null
+
+      - name: Ensure NixOS modules meta is valid
+        run: |
+          time nix-instantiate -I ./nixpkgs/untrusted --strict --eval --json ./nixpkgs/untrusted/nixos --arg configuration '{}' --attr config.meta --option restrict-eval true --option allow-import-from-derivation false