cmd/go: set up git identity for build_buildvcs_auto.txt

cmd/go: set up git identity for build_buildvcs_auto.txt

Just like in other tests like get_dotfiles.txt or
version_buildvcs_git.txt. Without it, I get a failure on my machine:

	fatal: empty ident name (for <mvdan@p14s.localdomain>) not allowed

Change-Id: I1c17c0d58c539b59154570b5438c7bd850bac5aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/416095
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>