Update Github action to use `pull_request_target` from https://docs.github.com/en/actions/reference/events-that-trigger-workflows#pull_request_target.
Update Github action to use `pull_request_target` from
https://docs.github.com/en/actions/reference/events-that-trigger-workflows#pull_request_target.

The existing behavior only triggered the action for PRs from the repo and not
PRs from forks, which made its usefulness minimal.

PiperOrigin-RevId: 325286854

tensorflow/model-optimization,tensorflow/model-optimization