/usr/lib/grub-rescue/grub-rescue-usb.img is in grub-rescue-pc 2.02~beta2-36ubuntu3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/grub-rescue/grub-rescue-cdrom.iso
/usr/lib/grub-rescue/grub-rescue-cdrom.iso