[GHF] Land validation should not change default branch (#84084)
This prevents a loophole, where somebody submits a PR that modifies merge rules and request land validation, so that their PR will be validated against those rules, rather than ones currently in trunk.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84084
Approved by: https://github.com/janeyx99, https://github.com/kit1980
