pacman-init.service: don't hardcode the keyring

pacman-init.service hardcodes the archlinux keyring. There should be no need to do that. If it's omitted and run as pacman-key --populate, it will use all of the keyrings from /usr/share/pacman/keyrings/.

This would simplify adapting releng for ISOs that ship with custom pacman keyrings.

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