Clean up baseline directories after each run

- baselines are deleted after all known out of date branches/PRs/tags
  have been built
- this limits the amount of reuse we can have, but if the build systems
  are busy, the baselines will be reused many times.

Perhaps not ideal, but now the build system fully cleans ups after
itself, which should solve our very real disk usage reliability problem.

@kbenne @Myoldmopar