ci: add formatting check

.github/workflows/cachix.yaml

@@ -9,6 +9,17 @@ on:
       - 'flake.lock'
       - '.github/workflows/*'
 jobs:
+  format-check:
+    name: Format check
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+
+    - uses: cachix/install-nix-action@v16
+
+    - name: Run alejandra
+      run: nix run nixpkgs#alejandra -- --check .
+
   flake-check:
     name: Flake check
     runs-on: ubuntu-latest
@@ -29,7 +40,7 @@ jobs:
   build-pkgs:
     name: Nix packages
     runs-on: ubuntu-latest
-    needs: [ flake-check ]
+    needs: [ flake-check, format-check ]
 
     strategy:
       fail-fast: false