cmd/go: fix test failures with -count=2

In each test, either set the -n flag to avoid writing build artifacts
to the cache, or set GOCACHE explicitly to point to a clean cache.

Tested manually with 'go test -count=2 cmd/go'.

Fixes #37820

Change-Id: I24403e738b1a10d5fe9dc8d98ef27a76ebe2704a
Reviewed-on: https://go-review.googlesource.com/c/go/+/223140


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>