ARROW-10063: [Archery][CI] Fetch main branch in archery build only when it is a pull request
ARROW-10063: [Archery][CI] Fetch main branch in archery build only when it is a pull request

Arrow's git data for the main branch is required to test the release curation scripts.

While the build properly works from pull requests it is failing on the main branch since the requested branch already exists: https://github.com/apache/arrow/runs/1148584775

Closes #8238 from kszucs/archery-build

Authored-by: Krisztián Szűcs <6bab32dc957a05a87930e779f272342e159326fa@gmail.com>
Signed-off-by: Krisztián Szűcs <6bab32dc957a05a87930e779f272342e159326fa@gmail.com>

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