fix(ci): fix packaging check merge operation (#5750)

github.sha does not always points to the last commit in the
source branch of a PR, leading to errors like the following one while
performing:

  $ git merge ${{ github.sha }}
  merge: d8432ed - not something we can merge

Use github.event.pull_request.head.sha instead.

See [1, 2] for more context:

- [1] https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows#pull_request
- [2] https://github.com/orgs/community/discussions/26325

.github/workflows/packaging-tests.yml

@@ -40,7 +40,7 @@ jobs:
         run: |
           git config user.name "GitHub Actions"
           git config user.email "actions@github.com"
-          git merge ${{ github.sha }}
+          git merge "${{ github.event.pull_request.head.sha }}"
 
       - name: Quilt patches apply successfully and tests run
         run: |