pkgctl: allow custom chroot dir

It would be great to allow functionality similar to how archbuild allows setting a custom directory for the chroots using the -r option:

archbuild -h
Usage: archbuild [options] -- [makechrootpkg args]
    -h         This help
    -c         Recreate the chroot before building
    -r <dir>   Create chroots in this directory

Default makechrootpkg args: -c -n -C

I am usually building all of my packages in tmpfs and without an override the default directory below /var/lib/archbuild is used. Currently tmpfs can only be used if that directory is mounted as tmpfs.

Alternatively, if this proves too difficult or unwanted, steps on how to build in tmpfs (using /var/lib/archbuild) would be great.