Set restrictive permissions on backup directories

Fixes a regression in !3569 (merged) it causes failures in gitlab-rails tests https://gitlab.com/gitlab-org/gitlab/-/jobs/1430950410

Ideally the test would have caught this failure, but depending on umask the directory already has such permissions. I'm not sure how best to fix the test, so I've left it alone for now.

This is blocking the gitaly update gitlab!66281 (closed)