net: fix lookupHost on Plan 9

net: fix lookupHost on Plan 9

CL 532217 added the newDNSError function.

However, the implementation was not correct on
Plan 9, which lead TestLookupNoSuchHost to fail.

This change fixes lookupHost on Plan 9.

Fixes #67096.

Change-Id: I39271f7d588b19c1b1608f18a24d871460be09cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/582236
Reviewed-by: Joedian Reid <joedian@google.com>
Run-TryBot: David du Colombier <0intro@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>