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