arch-nspawn: read CacheDir option from host's conf

This went unnoticed on build.archlinux.org until we tried switching away from its local /srv/ftp/ mirror. With a remote mirror, the chroots would ignore all the cached packages under /srv/ftp/pool/{packages,community}.

With the local file:// mirror gone, arch-nspawn wouldn't mount the cache directories from the host into the chroot. The fix is to read the option from the host's pacman.conf, instead of the one given to arch-nspawn and the one existing inside the working directory.