ci: run checks

.github/workflows/build.yml

@@ -3,6 +3,24 @@ on:
   pull_request:
   push:
 jobs:
+
+  checks:
+    strategy:
+      matrix:
+        check:
+          - nixpkgs-fmt
+          - deadnix
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: action/checkout@v3
+      - uses: cachix/install-nix-action@v19
+      - name: Check ${{ matrix.check }}
+        # Depends on nixos/nix#7759 to simply `nix flake check`
+        run:
+          - export SYSTEM=$(nix eval --raw --impure --expr "builtins.currentSystem")
+          - nix run .#checks.$SYSTEM.${{ matrix.check }}
+
   build-and-update:
     strategy:
       matrix: