debian: load skeletons after running debootstrap
debian: load skeletons after running debootstrap

Right now mkosi.skeleton cannot be used for dpkg-based distributions, since
debootstrap will not work on a non-empty target. This adds a parameter to
install_skeleton_trees to hack around this for Debian and Ubuntu, so that the
call before install_distribution is skipped and we only add skeletons before
invoking apt again after doing the initial debootstrap.

systemd/mkosi,systemd/mkosi