[9.20] fix: dev: Fix wrong variable in named_server_sync() log message

[9.20] fix: dev: Fix wrong variable in named_server_sync() log message

named_server_sync() logged isc_result_totext(result) but returns
tresult. The loop accumulates errors into tresult, so result only
holds the last iteration's value. If the last view succeeded but an
earlier one failed, the log would incorrectly say "success".

Backport of MR !12090

Merge branch 'backport-mnowak/fix-server-sync-log-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!12156