fix: usr: Fix trying the next primary server when the preivous one was marked as...

In some cases (there is evidence only when XoT was used) `named` failed
to try the next primary server in the list when the previous one was
marked as unreachable. This has been fixed.

Closes #5038

Merge branch '5038-xfr-primary-next-fix' into 'main'

See merge request isc-projects/bind9!9781