* [CIFuzz] Add functionality to save diskspace.
Add a LOW_DISK_SPACE env/config var. When this is specified
(always true for Github actions) run_fuzzers will delete
base-builder and the project builder image before fuzzing.
After it finishes fuzzing with a target, it will also
delete the targets, its seed corpus and its corpus.
Related: #4879
jonathanmetzman