fix: Check commit author instead of actor to detect release PR merges (#11633)
## Summary
Fixes the infinite release loop caused by merging release PRs with
auto-merge enabled.
- When a release PR with auto-merge is merged, `github.actor` is the
person who enabled auto-merge, not the bot
- The check now uses `github.event.head_commit.author.name` which
correctly identifies `github-actions[bot]` as the commit author
- This ensures the release workflow skips when merging release PRs
Related: https://github.com/vercel/turborepo/actions/runs/21590416087