ubuntu-server installer images contain extraneous /etc/apt/preferences.d.save directory

mwhudson@orcrist:~$ unsquashfs -lls ~/Downloads/livecd.ubuntu-server.ubuntu-server-minimal.ubuntu-server\ \(1\).squashfs | grep -F preferences.d
drwxr-xr-x root/root 70 2023-09-12 18:53 squashfs-root/etc/apt/preferences.d
-rw-r--r-- root/root 442 2023-09-12 18:53 squashfs-root/etc/apt/preferences.d/ubuntu-pro-esm-apps
-rw-r--r-- root/root 434 2023-09-12 18:53 squashfs-root/etc/apt/preferences.d/ubuntu-pro-esm-infra
drwxr-xr-x root/root 70 2023-09-12 18:53 squashfs-root/etc/apt/preferences.d.save
-rw-r--r-- root/root 442 2023-09-12 18:53 squashfs-root/etc/apt/preferences.d.save/ubuntu-pro-esm-apps
-rw-r--r-- root/root 434 2023-09-12 18:53 squashfs-root/etc/apt/preferences.d.save/ubuntu-pro-esm-infra

We create this directory with a chroot hook but clean it up with code in auto/build:

 if [ -d chroot/etc/apt/preferences.d.save ]; then
  # https://mastodon.social/@<email address hidden>
  mv chroot/etc/apt/preferences.d.save/* chroot/etc/apt/preferences.d/
  rmdir chroot/etc/apt/preferences.d.save
 fi

We need to do this potential clean up for each layer in lb_chroot_layered.