ci/ruff format: display diff when it would make changes (MR 2331)

.ci/ruff.sh

@@ -19,4 +19,4 @@ ruff check --ignore "F401" $(find . -not -path '*/venv/*' -name '__init__.py')
 ruff check --exclude=__init__.py .
 
 # Check formatting
-ruff format --check
+ruff format --diff