ci: tag the actual merge commit, not the test merge ref

Author: theomonnom

.github/workflows/publish.yml

@@ -196,6 +196,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+        with:
+          ref: ${{ github.event.pull_request.merge_commit_sha }}
 
       - name: Create git tag
         env: