Allow forked repos to be checked out for mapping

[KartikP]

Uses pull_request_target for both vision branches and forked vision branches while avoiding running untrusted code with secret access.

1. ref: ${{ github.base_ref }} checks out master from brain-score/vision regardless of internal vs fork PR. Ensures dependencies come from trusted code.

2. Existing Get Changed Files step continues to work for both. GitHub API knows about all PRs regardless of source.

3. Existing GitHub action expressions continue to work for both.