Fix labeler job

[cicdw]

I noticed that the labeler job was failing on contributor PRs (example here). I did some reading and found two things that needed fixing:

• we need to check out the repository so that the labeler.yml file could be found by the runner without an API call (it's not clear that this is critical but 🤷 )
• we need to use the pull_request_target event to enable write permissions back on the PR from forked PRs, which is safe in the context of a labeling job (see documentation on pull_request_target)

[cicdw]

why didn't the labeler job run on this PR? ...