all: use a hermetic "go" tool in standard-library tests

The go/build package uses the "go" tool from the user's environment,
but its tests should not assume that that tool is in any particular
state, let alone appropriate for running the test.

Instead, explicitly use testenv.GoTool, adding it to $PATH in a
TestMain when necessary.

Fixes #39199
Fixes #39198

Change-Id: I56618a55ced473e75dd96eeb3a8f7084e2e64d02
Reviewed-on: https://go-review.googlesource.com/c/go/+/234880


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>