gitlab_runner: Disable slow/unneeded systemd-time-wait-sync in the VM image

The service was enabled in arch-boxes to account for "hardware clock is
not in UTC, but instead UTC+X"[1], in our case the (VM) hardware clock
is in UTC and we therfor don't need the slow systemd-time-wait-sync
service (+30 seconds).

[1] archlinux/arch-boxes@e23d3c57