This patch (to /usr/lib/live/build/binary_rootfs) resolves the problem when building bookworm live isos on a bookworm host system. I don't know if it is right for other environments.
109,113c109,113 < if ! Chroot chroot "test -s /etc/mtab" < then < Chroot chroot "ln -s /proc/mounts/mtab /etc/mtab" < FAKE_MTAB=true < fi --- > # if ! Chroot chroot "test -s /etc/mtab" > # then > # Chroot chroot "ln -s /proc/mounts/mtab > /etc/mtab" > # FAKE_MTAB=true > # fi