chore: add gdformat check to flake

flake.nix

@@ -46,6 +46,30 @@
         }
       );
 
+      checks = forAllSystems (
+        { pkgs }:
+        {
+          gdformat = pkgs.stdenvNoCC.mkDerivation {
+            name = "gdformat-check";
+            src = pkgs.lib.sources.sourceFilesBySuffices (pkgs.lib.cleanSource ./.) [ ".gd" ];
+
+            nativeBuildInputs = with pkgs; [
+              gdtoolkit_4
+            ];
+
+            dontBuild = true;
+            doCheck = true;
+
+            checkPhase = ''
+              export HOME=$(mktemp -d)
+              find . -name "*.gd" -print0 | xargs -0 gdformat --check
+              echo "All .gd files are properly formatted"
+            '';
+            installPhase = "mkdir $out";
+          };
+        }
+      );
+
       packages = forAllSystems (
         { pkgs }:
         let