[release-branch.go1.11-security] 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: I6f343ae41ca7d984797e595867c8210b404b782f
Reviewed-on: https://team-review.git.corp.google.com/c/376740


Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>