nsupdate: fix zone lookup (#5818)
The SOA record for an existing zone is returned as an answer RR and not
as an authority RR. It can be returned as an authority RR for subdomains
of a zone.
$ dig -t SOA example.com
;; ANSWER SECTION:
example.com. 3530 IN SOA ns.icann.org. noc.dns.icann.org. 2022091184 7200 3600 1209600 3600
$ dig -t SOA www.example.com
;; AUTHORITY SECTION:
example.com. 3600 IN SOA ns.icann.org. noc.dns.icann.org. 2022091184 7200 3600 1209600 3600
(cherry picked from commit 5ad703ac64)
Co-authored-by: n0p90 <36303164+n0p90@users.noreply.github.com>
