ci: add formatting check (#30)

This will help enforce a consistent coding style. The formatter used is `nixpkgs-fmt`.

.github/workflows/ci.yml

@@ -0,0 +1,18 @@
+name: CI
+
+on:
+  - push
+  - pull_request
+
+jobs:
+  Check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: cachix/install-nix-action@v20
+        with:
+          github_access_token: ${{ secrets.GITHUB_TOKEN }}        
+      - name: Run formatter
+        run: nix run nixpkgs#nixpkgs-fmt .
+      - name: Check for changes
+        run: git diff --color=always --exit-code