Create binary_chroot stagefiles when skipping, too

Else, lb will error out when building with --build-with-chroot false:

[2023-07-18 12:00:15] lb binary --color
P: Setting up clean exit handler
[2023-07-18 12:00:15] lb binary_chroot --color
P: Begin copying chroot...
[2023-07-18 12:00:15] lb binary_rootfs --color
P: Begin building root filesystem image...
E: the following stage is required to be done first: binary_chroot
E: An unexpected failure occurred, exiting...

(We build our https://github.com/fsfw-dresden/usb-live-linux from inside a systemd-nspawn container with debian stable, so no chroot needed..)