Setting this variable will make xbps-src use an alternative mirror for
remote repositories.
Using this variable one can change the remote repository for all
architectures in a single setting and without the need to modify
`etc/xbps.d/repos-remote*.conf`. This is much more convenient as it
allows changing remote repos without a dirty worktree.
To use just add a line like the following to `etc/conf`:
XBPS_MIRROR=https://repo-us.voidlinux.org/current
We also disable 00-repository-main.conf for cross so we don't use the
remote repo from the xbps package.