fix: get author of pull request for PR review / PR review comment (#21278)

[Zulip thread (in reviewers stream)](https://leanprover.zulipchat.com/#narrow/channel/345428-mathlib-reviewers/topic/add.20author.20to.20.60maintainer-merge.60.20Zulip.20message)

Author: bryangingechen

.github/workflows/maintainer_merge.yml

@@ -19,7 +19,7 @@ jobs:
     # both set simultaneously: depending on the event that triggers the PR, usually only one is set
     env:
       AUTHOR: ${{ github.event.comment.user.login }}${{ github.event.review.user.login }}
-      PR_AUTHOR: ${{ github.event.issue.user.login }}
+      PR_AUTHOR: ${{ github.event.issue.user.login }}${{ github.event.pull_request.user.login }}
       PR_NUMBER: ${{ github.event.issue.number }}${{ github.event.pull_request.number }}
       COMMENT_EVENT: ${{ github.event.comment.body }}
       COMMENT_REVIEW: ${{ github.event.review.body }}