all: use testing.GoToolPath instead of "go"

all: use testing.GoToolPath instead of "go"

This change makes sure that tests are run with the correct
version of the go tool.  The correct version is the one that
we invoked with "go test", not the one that is first in our path.

Fixes #16577

Change-Id: If22c8f8c3ec9e7c35d094362873819f2fbb8559b
Reviewed-on: https://go-review.googlesource.com/28089
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>