Do not use a single static mirror for setting up build environments
Builds are sometimes failing on Jenkins because it is unable to connect to the Debian and Ubuntu mirrors at http://ftp.osuosl.org/pub/. For example: https://jenkins.torproject.org/job/tor-ci-linux-0.2.8/ARCHITECTURE=amd64,SUITE=wheezy/23/console
Debian has the HTTP mirror redirector which could solve the problem by automatically picking different mirrors. Ubuntu has a similar solution by using the global archive.