arch: Use gpgdir from host system

arch: Use gpgdir from host system

Instead of setting up the keyring in the image, let's reuse the
keyring from the host. If users want to use pacman in the image,
they just have to run pacman-key themselves in a postinst script
or such.

This speeds up building of images and hopefully also gets rid of
our CI issues with Arch where there's something keeping files open
in the root mount (which I expect is gpg-agent).