[release-branch.go1.21] cmd/go: set global .gitconfig location more robustly in tests

[release-branch.go1.21] 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.

For #53955.
For #64603.
Fixes #64586

Change-Id: I1a954975a3d8300e8b4dca045d3a15438a0407ec
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>
Reviewed-on: https://go-review.googlesource.com/c/go/+/585816
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>