cowspace can no longer be on same device as archiso files

Setting archisolabel and cow_label to the same device no longer works (presumably since d0d7eb25). It says, that it cannot mount the cowspace, since it is already mounted (full output can be seen here: https://ibb.co/vZkP7Sg).

Reverting to mkinitcpio-archiso-59-1 resolved the problem for me.