fix(ci): fix packaging check merge operation

[aciba90]

Proposed Commit Message

fix(ci): fix packaging check merge operation

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: d8432eda1ea1c722f6ff635f06c5f1d9c694a46f - 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

Additional Context

https://github.com/canonical/cloud-init/actions/runs/11067025369/job/30749489286?pr=5655

Test Steps

Merge type

• Squash merge using "Proposed Commit Message"
• Rebase and merge unique commits. Requires commit messages per-commit each referencing the pull request number (#<PR_NUM>)

[holmanb]

Thanks @aciba90!