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GH-39996: [Archery] Fix Crossbow build on a PR from a fork's main branch (#40002)

Instead of doing an intermediate bare clone, just fix the locally created branch name when fetching.

Amended from PR #39997, which wasn't sufficient (the `git fetch` that works for me doesn't seem to work on GHA).

* Closes: #39996

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>