"/in-pmbootstrap" file flag included in ondev installer image

Not a huge problem, but should be cleaned up nevertheless. I think clean_up() is being skipped when building the ondev installer image by this (create_device_rootfs calls it):

    if not args.ondev_no_rootfs:
        create_device_rootfs(args, step, steps)
        step += 1

https://gitlab.postmarketos.org/postmarketos/pmbootstrap/-/blob/master/pmb/install/_install.py#L811

cc @calebccff