ci: add merge_group trigger to ci tasks.

[cpu]

This commit adds merge_group to our CI task on triggers in preparation for enabling the merge queue feature.

Per the GitHub docs:

You must use the merge_group event to trigger your GitHub Actions
workflow when a pull request is added to a merge queue.

Note: If your repository uses GitHub Actions to perform required
checks on pull requests in your repository, you need to update the
workflows to include the merge_group event as an additional trigger.
Otherwise, status checks will not be triggered when you add a pull
request to a merge queue. The merge will fail as the required status
check will not be reported. The merge_group event is separate from the
pull_request and push events.

[cpu]

Thanks!