ci: add Nixpkgs lib-tests workflow

Ofborg also does this.
Should be run on `lib/*` dirs

Signed-off-by: John Titor <50095635+JohnRTitor@users.noreply.github.com>

.github/workflows/eval-lib-tests.yml

@@ -0,0 +1,30 @@
+name: "Building Nixpkgs lib-tests"
+
+permissions:
+  contents: read
+
+on:
+  pull_request_target:
+    paths:
+      - 'lib/**'
+jobs:
+  get-merge-commit:
+    uses: ./.github/workflows/get-merge-commit.yml
+
+  nixpkgs-lib-tests:
+    name: nixpkgs-lib-tests
+    runs-on: ubuntu-latest
+    needs: get-merge-commit
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        if: needs.get-merge-commit.outputs.mergedSha
+        with:
+          # pull_request_target checks out the base branch by default
+          ref: ${{ needs.get-merge-commit.outputs.mergedSha }}
+      - uses: cachix/install-nix-action@08dcb3a5e62fa31e2da3d490afc4176ef55ecd72 # v30
+        with:
+          # explicitly enable sandbox
+          extra_nix_config: sandbox = true
+      - name: Building Nixpkgs lib-tests
+        run: |
+          nix-build --arg pkgs "(import ./ci/. {}).pkgs" ./lib/tests/release.nix