bsdinstall: fix prepopulating the ZFS disk menu with ZFSBOOT_DISKS

If the ZFSBOOT_DISKS variable is set to one or more disk names, then
those disks should be preselected in the disk menu. However, the code
wasn't correctly setting the variable, leaving all disks unselected.

MFC after: 2 weeks
Sponsored by: Axcient
Reviewed by: dteske
Differential Revision: https://reviews.freebsd.org/D35331