Add support for GIT_CLEAN_FLAGS
The GitLab Runner today does a
git clean during the clone process after checkout, but hard coded to:
git clean -ffdx
In some scenarios, this might not be desirable (such as when building a large mono repo multiple times on the same runner), and the user may want to disable this functionality.
Additionally, users may want to customize this behavior with to exclude a certain pattern or other such methods available to
git clean: https://git-scm.com/docs/git-clean