arch-nspawn assumes that `file://` in pacman.conf always refers to host

Right now, arch-nspawn will iteratively search for file:// mirrors in repositories listed in the pacman.conf file for the chroot and assume that these are directories on the host to bind into the chroot. This is… a bad assumption, and it should be possible to turn it off.

As a simple example, I'm trying to build a set of custom packages for myself and would like to mount a repository of the built packages into the chroot, so that makepkg can use it to install dependencies if needed. I just set this directory up as /repo because I planned to always bind it when running makechrootpkg or arch-nspawn.

However, this makes arch-nspawn always fail, because it tries to be clever and mount in /repo from the host: which fails, because it does not exist on the host. It does not even check for the existence of the directory on the host and just adds it to the nspawn arguments, which fails because it cannot bind-mount a directory which does not exist.