cmd/go: accept GOSUMDB=sum.golang.google.cn

This CL makes the go command understand that
GOSUMDB=sum.golang.google.cn should connect
to that domain but expect to find a checksum database
signed by sum.golang.org there.

The host sum.golang.google.cn is not yet completely
configured; we hope it will be available in a few weeks.

Change-Id: Ie0fc4323f0c7084dda59bd3b45fc406717fa16d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/191137


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>