Switch to GeoIP mirror

Currently consists of 3 geographically diverse boxes which are managed
by DevOps. (See commit 9f65f99c6b2330e7e2fbceb988115c5cc65db09f in the
archlinux/infrastructure project.)

rootfs/etc/pacman.d/mirrorlist

-Server = https://mirror.pkgbuild.com/$repo/os/$arch
+Server = https://geo.mirror.pkgbuild.com/$repo/os/$arch
 Server = https://mirror.rackspace.com/archlinux/$repo/os/$arch
 Server = https://mirror.leaseweb.net/archlinux/$repo/os/$arch