Add basic evaluation check to run on pull requests

.github/workflows/pulls.yml

@@ -0,0 +1,17 @@
+name: Checks
+
+on:
+  pull_request:
+  push:
+
+jobs:
+  flake-show:
+    runs-on: ubuntu-latest
+    outputs:
+      matrix: ${{ steps.set-matrix.outputs.matrix }}
+    steps:
+      - uses: actions/checkout@v3
+      - uses: cachix/install-nix-action@v22
+      - id: set-matrix
+        name: Evaluate flake
+        run: nix flake show --all-systems