makemigrations --check as part of tests

I briefly considered making this a separate job, but I can't really justify that;
the 'separate' job would have to do all kinds of similar setup (pip install), and
to what end really? Broken is broken, I just want a notification. I'll see quickly
enought that it's about makemigrations

.github/workflows/ci.yml

@@ -51,6 +51,9 @@ jobs:
       with:
         repository: bugsink/event-samples
         path: "event-samples"
+    - name: Run Makemigrations --check
+      run: |
+        python manage.py makemigrations --check
     - name: Run Tests
       run: |
         python manage.py test -v2