release: ensure enforce_chs sysctl is 0

We do not want CHS-based alignment for VM or SD card release images.

(Other images use makefs/mkimg, not kernel gpart and so do not depend
on this sysctl.)

Sponsored by: The FreeBSD Foundation
Approved by: re (gjb)
MFC after: 1 week

(cherry picked from commit 6b6367ba8fd2b29de29ce08e7432291e807c3bc0)