Tweak cache-buildtrees option

This tweaks the recently merged cache-buildtrees option (#896 (closed), !1135 (merged)) based on a discussion on IRC.

• Drop support Add disclaimer for cache-buildtrees=never

  Not caching the buildtree for failed builds breaks basic core functionality such as dropping into a shell in the failed build sandbox.

• Rename cache-buildtrees option value failure to auto

  We anticipate other cases than build failures where buildtree caching will be required. E.g., incremental workspace build with remote execution. Or running tests in a buildtree in parallel with the build of reverse dependencies.

  This renames the option value failure to the more generic auto to cover these other cases as well.

• Change default of cache-buildtrees option to auto

  Caching buildtrees may significantly increase CAS disk usage. Change the default to cache buildtrees only when required for core functionality.