Don't share the pacman master-key between hosts

We shouldn't use the master-key from the Hetzner's snapshot but generate a new one on first boot.

arch-boxes is doing it with this service: https://gitlab.archlinux.org/archlinux/arch-boxes/-/blob/3508cf2c41204b03a461c637d809b9abc9cb71af/images/base.sh#L23-L38