go/importer,os/exec: use testenv.GoToolPath

go/importer,os/exec: use testenv.GoToolPath

These were the last two occurences of exec.Command("go", ...) in all of
std cmd. Checked with:

	gogrep '$(f is(func))("go", $*_)' std cmd

Also changed lp_windows_test to use a test package name to avoid a
circular dependency, since internal/testenv imports os/exec.

Change-Id: I9a18948600dfecc8507ad76172e219e78b791ffd
Reviewed-on: https://go-review.googlesource.com/87200
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>