[buildbot] Annotated builder tweaks
- Allow bypassing source code checkouts. Cloning complete LLVM tree takes 2-3 minutes and not all bots need it (e.g. some CUDA bots just need to run tests built somewhere else)
- Allow using out-of-tree annotated scripts. This is useful for tinkering with bot operations without having to update buildmaster.
Differential Revision: https://reviews.llvm.org/D83503