Do not go through the pmb.chroot.init() code path when running
pmb.install.losetup.umount() inside pmb.chroot.shutdown(). This is not
necessary, as pmb.install.losetup.umount() only gets called if the
chroot is already initialized and /dev/loop-control is mounted inside
the chroot.
Not going through this code path is important for the upcoming workdir
migration patch. Without this fix, it will fail with the following if
running "pmbootstrap install" before the work migration:
ERROR: Could not figure out on which release channel the 'native' chroot is
d3dc3b2c98