[TASK] gitlab-ci: Enable FF_ENABLE_JOB_CLEANUP flag

This gitlab-runner flag invokes a 'git clean' command
at the end of each job run, after uploading caches, to
remove any files that have been created by the job.

Until now, this cleanup has been done at the start
of the next job only, so all this temp data - around
half a gig to a gig per job - has been lying around
in the gitlab-runner docker cache volumes.

This is super useful since our runners work with a
ram driven var/lib/docker, this cleanup significantly
reduces ram usage on the runners.

Change-Id: I301d9ea24fab2c4c2202bd57c8e5ed6fd5f05fc0
Resolves: #95665
Releases: master, 10.4
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71807


Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>