dns/knot3: Workaround due to bug in service(8): /usr/sbin/service: ERROR: USAGE: load_rc_config_var name var

/usr/sbin/service: ERROR: USAGE: load_rc_config_var name var

This is very old and known bug in service(8) from 2016:
https://bugs.freebsd.org/208534
https://reviews.freebsd.org/D5833

PR: 284100
Approved by: Leo Vandewoestijne <freebsd@dns.company> (maintainer)
MFH: 2025Q1

(cherry picked from commit b7b2320af6f78ddc1a92a0a605ceee49183ab6fc)