// adbCopyGoroot clears deviceRoot for previous versions of GOROOT, GOPATH
// and temporary data. Then, it copies relevant parts of GOROOT to the device,
// including the go tool built for android.
// A lock file ensures this only happens once, even with concurrent exec
// wrappers.
func adbCopyGoroot() error {
	goTool, err := goTool()
	if err != nil {
		return err
	}
	cmd := exec.Command(goTool, "version")
	cmd.Stderr = os.Stderr