pacstrap: add an option to initialize an empty keyring in the new root

As I explained in archiso#190 (closed), the keyring on the host might have been initialized before the correct system time is set, resulting in future issues. As FS#62826 has been fixed, archlinux-keyring can now populate itself during installation. So, when -G is specified, instead of doing nothing, we can initialize an empty keyring and let pacman do the populating.

Also, I'm hoping that we can make this default in the future, which would fix the archiso issue without "automating and pre-configuring even more things" on their side :)