[#1266] .forgejo: Make 'fumpt' job fail on changed files

`gofumpt` always returns an exit code of 0, even when it finds
misformatted files. To make `fumpt` action behave as expected
we need to check if `gofumpt` changed any files.

Signed-off-by: Ekaterina Lebedeva <ekaterina.lebedeva@yadro.com>

.forgejo/workflows/tests.yml

@@ -106,4 +106,6 @@ jobs:
         run: make fumpt-install
 
       - name: Run gofumpt
-        run: make fumpt
+        run: |
+          make fumpt
+          git diff --exit-code --quiet