Pick up default mirror from environment variables `CROUTON_MIRROR_<distro>[_<arch>]`.

This is mainly useful in the context of automated testing, so these variables can be set to a local mirror in a `.bashrc` or something.

Can also be set by the user, but then `sudo -E` must be used to run crouton to make sure these variables get passed to the installer. So they'll probably never notice...

I looked for other ways of doing this in `test/run.sh`, but it would imply parsing parameters to crouton to detect which release is being installed, then detect the distribution from that, and finally checking that `-u` is not set (and possibly some other hacks I didn't think of). Doesn't look fun.