cmd/go: set global .gitconfig location more robustly in tests

cmd/go: set global .gitconfig location more robustly in tests

Also confirm that setting the location actually worked before
proceeding with the rest of the test.

This fixes a test failure with git versions older than 2.32.0.

Updates #53955.
Fixes #64603.

Change-Id: I1a954975a3d8300e8b4dca045d3a15438a0407ec
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/548215
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>