.github/workflows: fail if go generate creates a diff

TODO: treat that as a formatting step, and auto-run?
diff --git a/.github/workflows/go_test.yml b/.github/workflows/go_test.yml
index 4d72733..b736432 100644
--- a/.github/workflows/go_test.yml
+++ b/.github/workflows/go_test.yml
@@ -36,6 +36,12 @@
       - name: Go generate
         run: |
           go generate ./...
+          # if go generate created a diff
+          if ! git diff --exit-code; then
+            echo "go generate created a diff"
+            git diff
+            exit 1
+          fi
 
       - name: Run tests
         run: |