Checkout PR branches with a merge commit for CI tests

[albertvillanova]

This PR runs the CI tests with a merge commit of PR branches.

TODO:

• Remove this requirement user-side
  • I have no access to the repo settings:
    • Settings > Branches > Branch protection rules > main > Edit
      • Require status checks to pass before merging:
        • Uncheck: Require branches to be up to date before merging

[HuggingFaceDocBuilderDev]

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