cmd/go: fix TestScript/govcs

cmd/go: fix TestScript/govcs

On my Mac, TestScript/govcs was failing because hg prints a URL
using 1.0.0.127.in-addr.arpa instead of 127.0.0.1, and my Mac
cannot resolve that name back into an IP address.

% host 127.0.0.1
1.0.0.127.in-addr.arpa domain name pointer localhost.
% host 1.0.0.127.in-addr.arpa
%

Change-Id: Ia0762342d5926d13d786fe66de40590dc8977ff5
Reviewed-on: https://go-review.googlesource.com/c/go/+/718184
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@google.com>
Auto-Submit: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>