Allow the GHA to report the unit tests results

See:
https://github.com/DOMjudge/domjudge/actions/runs/12247242151/job/34164708661?pr=2879

Here the push from dependabot failed to add the check-run results. Other
possible permissions:
https://docs.github.com/en/rest/checks/runs?apiVersion=2022-11-28#create-a-check-run

.github/workflows/unit-tests.yml

@@ -18,6 +18,8 @@ jobs:
         run: diff newcodecov .github/jobs/uploadcodecov.sh
 
   unit-tests:
+    permissions:
+      checks: write
     runs-on: ubuntu-24.04
     timeout-minutes: 20
     container: