Action doesn't work for forks (Resource not accessible by integration)

[TimonVS]

The Action fails to run on PRs that are submitted from a fork. As an example see the failing check on #24. It's basically the same issue as actions/labeler#12.

[JohnRDOrazio]

I was able to implement for PR's coming from forks by using the pull_request_target action instead of the pull_request action:

name: PR Labeler
on:
  pull_request_target:
    types: [opened]