Allow MIRRORSITE to pass sudo
Allow MIRRORSITE to pass sudo

cowbuilder always passes a --mirror.  When one system is used to build both Debian and Ubuntu packages, it is necessary to specify a MIRRORSITE for at least some of them.  This permits it to cross sudo.
mika/jenkins-debian-glue,linuxmaniac/jenkins-debian-glue,hashar/jenkins-debian-glue,mika/jenkins-debian-glue,linuxmaniac/jenkins-debian-glue,hashar/jenkins-debian-glue