release: Consistently use variables for invoking certain tools

This change cleans up the inconsistent ways of invoking makefs and mkimg
by making sure that these tools are invoked through their corresponding
variables. This was previously only used in a few amd64 and arm64 release
scripts.

Sponsored by: Klara, Inc.
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51039
Reviewed by: emaste, markj