Point binder to the branch

[fcollonval]

I think it will make more sense to spin binder on the PR branch than on a PR commit.

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[consideRatio]

I think this makes perfect sense, given that this comment workflow is triggered on pull_request_target such as "Opened" rather every time there is a new commit. It would also make sense to make it only trigger on "Opened" rather than all of "opened", "assigned", "reopened", etc.

Thank you @fcollonval for contributing this improvement!

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