To save time when performance testing, the tool will cache builds of YUI to prevent unnecessary cloning/rebuilding.  The current flow in the performance testing tool is to:
  1. Clone the local `yui3` repo to an OS (or user) specified temp directory
  1. Rebuild (if it isn't a version tag)
  1. Move built files to `yui3/.builds/{ref}-{sha}/`
  1. Let the OS cleanup the temp files when it wants to

For example:

    $ ls -1 .builds/
    v3.11.0-d549e5ca7e1827182fce393d843c3ba00f56c712
    v3.8.0-d89374d7213ad8260e5004200e8f99efd54e705b

These cached builds should be stored someplace common for all YUI tools to use, if needs arise in the future.  Options are either `~/.yui3-builds` or `yui3/.builds`.  I don't think it matters to me either way as I believe the home directory can be derived in an OS-agnostic way.