[stronghold][bc-linter] correctly determine the base commit of the PR (#98538)
Currently `${{ github.event.pull_request.base.sha }}` returns the HEAD of the base branch, which is different from **the base of the PR**.
See:
https://github.com/github/docs/issues/431
https://github.com/orgs/community/discussions/39880
However, BC linter needs to know the base revision **of the PR**, as it looks at the changes **in the PR**.
This change is a workaround that determines the correct base of the PR. Hopefully in the future GH provides this information in the event, and this workaround could be removed.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98538
Approved by: https://github.com/PaliC
