cmd/go: set user and email in test-local git repos

Some of the builders cannot infer user and email from the builder hostname.

Change-Id: I27e5d011fa1471f27763b6b7fa1bf59e418b925c
Reviewed-on: https://team-review.git.corp.google.com/c/376739


Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-on: https://go-review.googlesource.com/c/154107


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>