update_screenshots manual job fails on MRs from forks

The update_screenshots manual job fails when:

• The source branch is from a fork of GitLab UI
• The author of the MR has allowed collaboration with upstream members

Of course the job would also fail if the MR author did not allow collaboration with upstream members, but that should be expected, and isn't really a problem.

It appears that the push that the GitLab Bot user attempts to make fails; perhaps because it's not a member of the upstream project?

Example failure: https://gitlab.com/gitlab-org/gitlab-ui/-/jobs/1288349564 (from !2174 (merged))

This makes accepting community contributions which touch/update visual snapshots in some way a bit more tedious, either by temporarily breaking master, or by recreating the MR from the upstream repo.