release: Don't install missing lib32 into VMs

There is no FreeBSD-set-lib32 on riscv64 or powerpc64le.

Reviewed by: ivy, emaste
Fixes: 2d184465e8bb ("release: Rework VM package selection using sets")
MFC after: 6 hours
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D52987

(cherry picked from commit 0dc634d48fcca9e88c3fb12cdffab4ddbc657f99)