freebsd-update: do not restart sshd when updating jail/basedir

In 6cd1bc531609 for PR 263489 I changed freebsd-update to restart sshd
after upgrade, to avoid an upgrade-related incompatibility that made it
impossible to login.

This is intended to avoid losing access to remote hosts, and ought not
apply to upgrading jails (from outside).

PR: 263489, 272282
Reported by: otis
Reviewed by: otis, kevans
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Fixes: 6cd1bc531609 ("freebsd-update: restart sshd after upgrade")
Differential Revision: https://reviews.freebsd.org/D41890