ci/lib-tests: init

This allows running the lib-tests locally in exactly the same way that
they are run in CI:

  nix-build ci -A lib-tests

.github/workflows/lib-tests.yml

@@ -28,4 +28,4 @@ jobs:
 
       - name: Building Nixpkgs lib-tests
         run: |
-          nix-build --arg pkgs "(import ./ci/. {}).pkgs" ./lib/tests/release.nix
+          nix-build ci -A lib-tests

ci/default.nix

@@ -70,4 +70,7 @@ in
   requestReviews = pkgs.callPackage ./request-reviews { };
   codeownersValidator = pkgs.callPackage ./codeowners-validator { };
   eval = pkgs.callPackage ./eval { };
+
+  # CI jobs
+  lib-tests = import ../lib/tests/release.nix { inherit pkgs; };
 }