Fix the labeler workflow permission issue when merging from fork repo.
It impacts the labeler workflow to support auto-merge for package versions upgrade on 202012 branch. The current workaround is to add the label "automerge" on the PR sent by mssonicbld, then the automerge workflow will merge the PR.
Support to merge the SONiC package versions upgrade pull request automatically.
#### How I did it
Add the automerge tag in the pull request.
1. Sent by the author mssonicbld
2. Only contains changes in files/build/**/*
3. Only for the specified branch 202012
Merge the pull request.
1. Sent by the author mssonicbld
2. The Azure Pipelines checker complete and successful
3. The pull request contains label automerge
