add more tests

.forgejo/workflows/nix-tests.yaml

@@ -0,0 +1,22 @@
+name: Test Nix flake
+
+on:
+  push:
+  workflow_dispatch:
+
+jobs:
+  nix:
+    runs-on: nix-latest
+    name: Test nix flake
+    steps:
+      - name: Checkout repository
+        run: git clone ${{ env.FORGEJO_SERVER_URL }}/${{ env.FORGEJO_REPOSITORY }} .
+      - name: Run formatter
+        run: nix fmt
+      - name: Check flake
+        run: nix flake check
+      - name: Show metadata
+        run: nix flake metadata
+      - name: Show content
+        run: nix flake show
+

flake.nix

@@ -23,7 +23,7 @@
     in
     {
       formatter.${system} = pkgs.nixfmt-tree;
-      apps = {
+      apps.${system} = {
         nixos-diff = {
           type = "app";
           program = "${pkgs.writeShellScript "nixos-diff.sh" ''