Use geo.mirror.pkgbuild.com as the default mirror

geo.mirror.pkgbuild.com is a GeoDNS mirror that points to the sponsored mirrors.

This avoids the usage of Arch's own infrastructure (mirror.pkgbuild.com) for large downloads.
See archlinux/infrastructure!522 for details.
6 jobs for geo-mirror in 10 minutes and 21 seconds (queued for 3 seconds)