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GH-39996: [Archery] Fix Crossbow build on a PR from a fork's main branch (#39997)



### Rationale for this change

### What changes are included in this PR?

### Are these changes tested?

### Are there any user-facing changes?

* Closes: #39996

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