Baseline: 8b453331

Cherry picks:

   + 223113c9:
     Automated rollback of commit
     ae2a6a2d.

Important changes:

  - Adds --experimental_reuse_sandbox_directories flag to reuse
    already-created non-worker sandboxes with cleanup.
  - --experimental_force_gc_after_build is deprecated and will be
    removed soon. Use --bep_publish_used_heap_size_post_build instead
  - Forward coverage-instrumented files from non-tool dependencies by
    default.

This release contains contributions from many people at Google, as well as David Ostrovsky, Johannes Abt, Masoud Koleini, Pras Velagapudi.