Get the base ref also for the merge_group

Author: jasagredo

.github/workflows/checks.yml

@@ -31,7 +31,7 @@ jobs:
 
     - uses: actions/checkout@v3
       with:
-        ref: ${{ github.event.pull_request.base.ref }}
+        ref: ${{ github.event.pull_request.base.ref || github.event.merge_group.base_ref }}
         path: 'main'
 
     - run: CI=1 ./this-pr/scripts/ci/check-changelogs.sh main this-pr