sysutils/arc_summary: Match script to FreeBSD version

The latest arc_summary from the OpenZFS repository fails to run on
FreeBSD 13.2. Resolve this by installing arc_summary from a branch of
our src repository that matches the major version of the target system.

PR: 274605
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D43070