ci: allow parallel merge queue builds [WPB-9914]

[vitorhugods]

WPB-9914 Merge Queues - Use them to their fullest

---

PR Submission Checklist for internal contributors

• The PR Title

  • conforms to the style of semantic commits messages¹ supported in Wire's Github Workflow²
  • contains a reference JIRA issue number like SQPIT-764
  • answers the question: If merged, this PR will: ... ³

• The PR Description

  • is free of optional paragraphs and you have filled the relevant parts to the best of your ability

---

What's new in this PR?

Issues

Current concurrency strategy doesn't allow multiple builds in parallel for the merge queue.

Causes

When running the merge queue, the github.event.pull_request is empty. So it means that builds from multiple PRs will have the same concurrency group.

Solutions

If the PR number is null, use the merge_group.head_sha.

Testing

N/A

---

PR Post Merge Checklist for internal contributors

• If any soft of configuration variable was introduced by this PR, it has been added to the relevant documents and the CI jobs have been updated.

---

References

1. https://sparkbox.com/foundry/semantic_commit_messages
2. https://github.com/wireapp/.github#usage
3. E.g. feat(conversation-list): Sort conversations by most emojis in the title #SQPIT-764.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[github-actions]

Built wire-android-staging-release-pr-3140.apk is available for download

[github-actions]

Built wire-android-dev-debug-pr-3140.apk is available for download