arch-nspawn should use the correct pacman config file
Created by: deep-42-thought
If arch-nspawn is called with -C, pacman inside the chroot will use the provided configuration file. This should also be the case for $pacconf_cmd and pacman outside the chroot.
If arch-nspawn is called without -C, pacman inside the chroot will use $workdir/etc/pacman.conf -- again, $pacconf_cmd and pacman outside the chroot should use that, too. So lets just set $pac_conf in that case.
For example, Arch Linux 32 provides separate pacman configurations inside /usr/share/devtools which use /etc/pacman.d/mirrorlist32 as mirrorlist for their build commands (extra-i686-build, etc.). This way, we can build i686 and x86_64 packages on the same x86_64 host with very minimal changes to devtools.