Add GH action to verify fmtting

.github/workflows/validate.yaml

@@ -0,0 +1,12 @@
+name: Validate if the Nix setup is correctly formatted and builds
+on: [push]
+jobs:
+  check-if-formatted:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: cachix/install-nix-action@v22
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+    - uses: actions/checkout@v3
+    - name: Check format
+      run: nix-shell -p nixpkgs-fmt --run 'nixpkgs-fmt --check .'

flake.nix

@@ -11,6 +11,7 @@
       let
         pkgs = nixpkgs.legacyPackages.${system};
         isDarwin = pkgs.stdenv.isDarwin;
+        # thanks to https://github.com/commercialhaskell/stack/issues/1698#issuecomment-178098712 for the idea
         darwinFrameworks = pkgs.lib.optionals isDarwin (with pkgs.darwin.apple_sdk.frameworks; [
           Cocoa
           CoreServices