linting: output diffs from black

Just the message from --check is not enough to locate the problem manually.
Output the diffs for PR submissions where the submitter may not necessarily
have black installed.

.github/workflows/linting.yml

@@ -20,4 +20,4 @@ jobs:
       - name: isort
         run: isort --recursive --diff .
       - name: Black
-        run: black --check .
+        run: black --check --diff .