Allow makechroot options in archbuild

This will allow, by example, to easily build a package with a custom pkg.

staging-x86_64-build -- -cI /var/cache/pacman/pkg/ldoc-1.2.0-1-any.pkg.tar.xz
staging-x86_64-build -- -n

Signed-off-by: Sébastien Luttringer <seblu@seblu.net>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>