Run Playground Preview build against latest PR, not latest main

[carstingaxion]

Fix ref, to checkout before build, to point to the latest PR commit, not the latest commit of the main branch.

Description of the Change

Closes # Testing & Clean-up after #666, #741, #749, #750, #753, #757 & #770

How to test the Change

Changelog Entry

Added - New feature
Changed - Existing functionality
Deprecated - Soon-to-be removed feature
Removed - Feature
Fixed - Bug fix
Security - Vulnerability

Credits

Props @username, @username2, ...

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[what-the-diff]

PR Summary

• Change in the Reference of The Checkout Step
  The reference point in the 'Checkout' phase of the 'playground-preview.yml' workflow has been modified. Instead of using the current GitHub reference, it will now use the unique identifier (SHA) of the latest commit from the pull request. This ensures the correct version of the code is being processed in this workflow.

[github-actions]

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