buildbot: cut the virtualenv setup times
$PIP_DOWNLOAD_CACHE http://stackoverflow.com/questions/4806448/how-do-i-install-from-a-local-cache-with-pip
(from redmine: created on 2013-01-24, closed on 2013-01-31)
$PIP_DOWNLOAD_CACHE http://stackoverflow.com/questions/4806448/how-do-i-install-from-a-local-cache-with-pip
(from redmine: created on 2013-01-24, closed on 2013-01-31)