[wip] [ci] doc-job-skip take #4.5 dry-run via github direct edit
This is take 4.5 on attempting to find a reliable way to get a list of modified files of this PR. It's identical to https://github.com/huggingface/transformers/pull/8980 but this PR was created from github UI direct file edit, so as we can see, it doesn't provide `CIRCLE_PR_NUMBER` - Nothing bad happens, but the check can't be done  since we have no information to work with :( 

It also happens with PR's made from a non-personal branch, https://github.com/huggingface/transformers/pull/9015
And the result is that the check is completely skipped as it has no data to work with:
https://app.circleci.com/pipelines/github/huggingface/transformers/17118/workflows/48285d78-cb04-4feb-87f8-77cb02ac2593/jobs/134493

Hoping that circlePR will fix that bug.

This PR:

* [x] tests a PR submission from non-personal forked repo
* [x] switches to `head.user.login` for the username to checkout the branch with - using PR username as it's in the master will not work if the branch is coming from a non-forked repo (original that is). (could also use `.head.repo.full_name` for the whole thing at once.)

For now I will let this PR sit for a while and add other fixes if we find more edge cases.
