ci: test NixOS' VM test for PostgREST

This adds a CI job to run our latest postgrest version against the NixOS
VM test currently available in Nixpkgs. Now, this will not always be in
sync, so has the potential to be failing. However, since the Nixpkgs VM
test is really simple, this should only happen when we introduce a
breaking change on a very fundamental level. The failing test will then
be resolved once the new version is available in Nixpkgs and we have
updated our lock file.

This is essentially just a sanity check to make sure we're not breaking
something fundamentally - and if we do, it's a head up for me to adjust
the Nixpkgs tests accordingly. Those might otherwise break unnoticed
since Nixpkgs does not have a good notification system for such
breakages in place.

We do use the chance to run the static executable in this test, which
was previously not tested at all.

It also gives us a first test whether NixOS VM tests work well in GitHub
Actions.

.github/workflows/build.yaml

@@ -68,6 +68,11 @@ jobs:
           path: postgrest-docker-${{ matrix.artifact }}.tar.gz
           if-no-files-found: error
 
+      - name: Test static executable with NixOS' VM test
+        # GHA's ARM runner does not support KVM
+        if: runner.arch == 'X64'
+        run: nix-build -A nixpkgs-nixos-test
+
 
   macos:
     name: Nix - MacOS