// DownloadZip uses the same lockfile as Download.
	// Invoke DownloadZip before locking the file.
	zipfile, err := DownloadZip(ctx, mod)
	if err != nil {
		return "", err
	}

	unlock, err := lockVersion(ctx, mod)
	if err != nil {
		return "", err
	}
	defer unlock()

	ctx, span = trace.StartSpan(ctx, "unzip "+zipfile)
	defer span.Done()