Fix dependabot coverage exclusion

See https://docs.github.com/en/actions/reference/workflows-and-actions/events-that-trigger-workflows#pull_request

For PRs `github.ref` is actually `refs/pull/PULL_REQUEST_NUMBER/merge`.

Author: kelunik

.github/workflows/maven.yml

@@ -148,7 +148,7 @@ jobs:
     # Do not run coverage for forks since they cannot upload
     # the results to codecov. For reference, see:
     # https://docs.github.com/en/actions/using-jobs/using-conditions-to-control-job-execution#example-only-run-job-for-specific-repository
-    if: github.repository == 'casid/jte' && !startsWith(github.ref, 'refs/heads/dependabot/')
+    if: github.repository == 'casid/jte' && !startsWith(github.head_ref, 'dependabot/')
     runs-on: ubuntu-latest
     needs:
       - test-gradle-plugin