ci: use syntax of pull ref branch merge

Author: zimeg

.github/workflows/tests.yml

@@ -86,7 +86,7 @@ jobs:
     name: Regression notifications
     runs-on: ubuntu-latest
     needs: build
-    if: failure() && github.ref == 'refs/heads/zimeg-ci-notifications' && github.event_name != 'workflow_dispatch'
+    if: failure() && github.ref == 'refs/pull/1323/merge' && github.event_name != 'workflow_dispatch'
     steps:
       - name: Send notifications of failing tests
         uses: slackapi/slack-github-action@b0fa283ad8fea605de13dc3f449259339835fc52 # v2.1.0