bsdinstall: disable SUJ by default

SUJ + cg checksums produce spurious warnings after an unclean shutdown
(e.g. a crash). For now disable SUJ for UFS installs so installing from
a FreeBSD 12 snapshot results in a usable filesystem, until this can be
fixed.

PR: 225110
Submitted by: Arshan Khanifar <arshankhanifar gmail.com>
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D13890