flake.nix: add formatter check

.github/workflows/test.yml

@@ -15,13 +15,15 @@ jobs:
     - uses: cachix/install-nix-action@v31
       with:
         nix_path: nixpkgs=channel:nixos-unstable
+        extra_nix_config: |
+          experimental-features = nix-command flakes
     - run: |
         if grep -R --exclude stdlib-extended.nix literalExample modules ; then
           echo "Error: literalExample should be replaced by literalExpression" > /dev/stderr
           exit 1
         fi
     - run: nix-build --show-trace -A docs.jsonModuleMaintainers
-    - run: ./format -c
+    - run: nix flake check
     - run: nix-shell --show-trace . -A install
     - run: yes | home-manager -I home-manager=. uninstall
     - run: nix-shell -j auto --show-trace --arg enableBig false --pure tests -A run.all

flake.nix

@@ -56,6 +56,10 @@
           };
         });
     in {
+      checks = forAllSystems   (system: {
+        formatting = treefmtEval.${system}.config.build.check self;
+      });
+
       formatter = forAllSystems  (system: treefmtEval.${system}.config.build.wrapper);
 
       packages = forAllSystems (system: