Use existing commitId for tarball URL in stats action (#92899)
## Summary
PR #90709 introduced a `githubHeadSha` field in the stats action using
`event.after || GITHUB_SHA`, which can resolve to the wrong SHA for PR
events. The correct value was already available as `commitId` (from
`getCommitId()` = `git rev-parse HEAD` on the cloned PR branch).
- Use `commitId` for the tarball URL in `add-comment.js`
- Remove the redundant `githubHeadSha` field from `action-info.js`
## Test plan
- [ ] Verify the stats comment on a PR shows the correct tarball URL
matching the deployed artifact
