cmd/go: use build cache for tests when GOCACHE is unset

Before this CL, if you had GOCACHE=/some/dir, then the cmd/go tests used it.
But if you were relying on the implicit behavior that GOCACHE being empty
meant an appropriate system-specific cache directory, then the cmd/go tests
ran with no cache at all, which makes them about 4X slower.

During all.bash GOCACHE is set to a fresh temporary directory and is therefore
already getting proper caching; this CL mainly helps people running 'go test cmd/go'
by hand.

Change-Id: I7c322ca79b877c1d0a3b448b95d5354fbfcba7f8
Reviewed-on: https://go-review.googlesource.com/118320


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>