release: Use unprivileged mode for bootonly and mini-memstick

This in effect always builds these targets in NO_ROOT mode, removing
one more instance where release artifact builds require a privileged
user (root). These artifacts are now always built the same way.

Reviewed by: brooks, Pat Maddox <pat@patmaddox.com>
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48121