common: use tier0 mirror on build servers + gemini

There is a need for build servers to never build against outdated repo
databases, even with syncrepo providing a local mirror that is updated
every minute. To that effect, we adjust mirrorlist on build servers so
the first mirror is the tier0 mirror provided by gemini.

Keep the syncrepo role on build servers in order to have a local cache
of packages and avoid concurrent build jobs downloading the same files
causing them to be corrupted.

Finally, configure gemini to use its own repos (like other mirrors do).