Follow-up from "Add Ruff as a linter and formatter with pre-commit"

The following discussion from !1315 (merged) should be addressed:

• @alatiera started a discussion: (+5 comments)

  why is this needed? and why is it a path under root?

  Looks like pre-commit will use XDG_CACHE_HOME if it's set also.

  if we really need to, we could set it as $CI_PROJECT_DIR/pre-commit-home