Draft: Prevent deleting working directory and restoring bootstrap after lb clean --binary

According to the manual at 4.6, you can rebuild a hybrid iso to a hdd image by running:

lb clean --binary; lb config -b hdd; lb build

This fails because the working directory is deleted at bootstrap_cache restore. At first run, there was no bootstrap yet, so no bootstrap_cache restore was run and therefore no stagefile was created. At the second run, bootstrap_cache restore checks if it was run and deletes the working directory/chroot and restores the bootstrap. A workaround is to cache the chroot stage but it takes longer, takes up much more space and is not necessary according to the manual. Technically if bootstrap has run, and the stage file is not removed, it should not be restored.