Fix check-labels workflow commenting on forked PRs (#101467)
Using `pull_request_target` allows securely passing the secrets to make comments on a forked PRs.
See more about `pull_request_target` in https://github.blog/2020-08-03-github-actions-improvements-for-fork-and-pull-request-workflows/
The change was verified in https://github.com/malfet/deleteme/pull/53 - with `on pull_request` there were no "This PR needs a label" comment, with with `on pull_request_target` the comment can be posted.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101467
Approved by: https://github.com/malfet
