[9.16] Suppress Git clean output in GitLab CI

Merged Michal Nowak requested to merge mnowak/suppress-gitlab-clean-up-output-9.16 into bind-9.16

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).

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