[CI] fix git errors related to directory ownership

[jameslamb]

The Test R package on Debian CI job is currently failing on master (build link) and on #8627 (build link) with the following error at the step where the repo is git clone'd.

ValueError: ('Failed to check git repository status.', CompletedProcess(args=['git', 'clean', '-xdf', '--dry-run'], returncode=128, stdout=b'', stderr=b"fatal: detected dubious ownership in repository at '/__w/xgboost/xgboost'\nTo add an exception for this directory, call:\n\n\tgit config --global --add safe.directory /__w/xgboost/xgboost\n"))

I suspect this means that that job started getting a newer version of git recently, which contains a patch for the CVE described at https://github.blog/2022-04-12-git-security-vulnerability-announced/.

This PR proposes a patch that I think will address it. We've been using something similar over in LightGBM for the last 8 months (microsoft/LightGBM#5152) without issue.

[hcho3]

@jameslamb Thanks a ton!