ci/basic-eval: check that flake outputs are valid

.github/workflows/basic-eval.yml

@@ -26,5 +26,6 @@ jobs:
         # This cache is for the nixpkgs repo checks and should not be trusted or used elsewhere.
         name: nixpkgs-ci
         signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
+    - run: nix --experimental-features 'nix-command flakes' flake check --all-systems --no-build
     # explicit list of supportedSystems is needed until aarch64-darwin becomes part of the trunk jobset
     - run: nix-build pkgs/top-level/release.nix -A release-checks --arg supportedSystems '[ "aarch64-darwin" "aarch64-linux" "x86_64-linux" "x86_64-darwin"  ]'