ci: ensure flake is formatted

.github/workflows/nix.yml

@@ -0,0 +1,22 @@
+name: Test Nix flake
+
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+jobs:
+  format:
+    name: check flake formatting
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          show-progress: false
+      - uses: DeterminateSystems/nix-installer-action@main
+      - uses: DeterminateSystems/magic-nix-cache-action@main
+      - run: nix fmt
+
+      # Check that formatting does not change anything.
+      - run: git diff --exit-code

flake.nix

@@ -533,12 +533,14 @@
               };
           };
 
-        devShells.default = let 
+        devShells.default =
+          let
             pkgs = import nixpkgs {
               system = system;
               overlays = [ fenix.overlays.default ];
             };
-          in pkgs.mkShell {
+          in
+          pkgs.mkShell {
 
             buildInputs = with pkgs; [
               (fenix.packages.${system}.complete.withComponents [