Update playground workflow to run only on pull requests

[renatho]

Proposed Changes

• I noticed through https://github.com/Automattic/sensei/actions/runs/8172132019/job/22342087087 that I broke the trunk workflow with Add Sensei to the allowed repositories for plugin proxy WordPress/wordpress-playground#1079.
• I tried to extract the playground job to another workflow, but I wasn't able to make it work. For some reason, I couldn't use the peter-evans/create-or-update-comment@v4 when using the event workflow_run to run based on the Plugin Build. So I just added a new job with the conditional to run only in pull_requests.

Testing Instructions

1. Check that the Playground comment continues being added.
2. And after merged, see that the workflow on trunk works properly.

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Acceptance criteria is met
• Decisions are publicly documented
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• New UIs match the designs
• Different user privileges (admin, teacher, subscriber) are tested as appropriate
• Legacy courses (course without blocks) are tested
• Code is tested on the minimum supported PHP and WordPress versions
• User interface changes have been tested on the latest versions of Chrome, Firefox and Safari
• "Needs Documentation" label is added if this change requires updates to documentation
• Known issues are created as new GitHub issues

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.79%. Comparing base (8281657) to head (3792c6a).
Report is 1 commits behind head on trunk.

Additional details and impacted files

@@            Coverage Diff            @@
##              trunk    #7527   +/-   ##
=========================================
  Coverage     51.79%   51.79%           
  Complexity    11239    11239           
=========================================
  Files           622      622           
  Lines         47520    47520           
  Branches        414      414           
=========================================
  Hits          24613    24613           
  Misses        22574    22574           
  Partials        333      333           

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4442766...3792c6a. Read the comment docs.

[github-actions]

Test the previous changes of this PR with WordPress Playground.