This simplifies boot loader configuration, but custom PXE setups will be forced to update theirs. Implements https://gitlab.archlinux.org/archlinux/archiso/-/issues/226
See https://www.supergrubdisk.org/wiki/Loopback.cfg for details. Only `${iso_path}` is guaranteed, so we need to search for the volume, on which the ISO file resides, ourselves. Implements https://gitlab.archlinux.org/archlinux/archiso/-/issues/165