[9.18] Suppress Git clean output in GitLab CI
Git clean in GitLab CI can take up to a thousand lines, thus cluttering the output for no good reason.
The default GitLab GIT_CLEAN_FLAGS variable is extended with "q" as in "quiet.
(cherry picked from commit ddb33a12)
Backport of MR !7702 (merged).
Edited by Michal Nowak