This is so that users calling apt-get install from a hook only need to
have APT_CONFIG=$MMDEBSTRAP_APT_CONFIG set and do not also have to pass
-oDPkg::Chroot-Directory="$1".
This breaks users running apt-get with
DPkg::Options::=--force-script-chrootless or with Dpkg::Pre-Install-Pkgs
from within a hook with APT_CONFIG=$MMDEBSTRAP_APT_CONFIG.
In those situations, DPkg::Chroot-Directory has to be set to the empty
string explicitly with -o to overwrite the APT_CONFIG setting.
Thanks: Helmut Grohne
