(RE-2157) Add support for arbitrary deb mirrors to packaging

This commit leverages the work in a recent change to the pbuilderrc to
allow for overriding the secondary mirror to build against. It adds a
new build parameter to Pkg::Config, deb_build_mirrors, which should be
an array of deb sources to inject into the build using the BUILDMIRROR
environment variable. The dist can be templated in using __DIST__.
(RE-2157) Add basic tests for the set_cow_envs method

As the previous commit updated the behavior of the set_cow_envs utility
method, this commit adds some tests for the new and for the existing
behavior.
Also reset the ENV after the tests are run