code: func UserRoot() string {
  	//  pam_systemd sets XDG_RUNTIME_DIR but not other dirs.
  	xdgDataHome := os.Getenv("XDG_DATA_HOME")
  	if xdgDataHome != "" {
  		dirs := strings.Split(xdgDataHome, ":")
  		return filepath.Join(dirs[0], "buildkit")
  	}
  	home := os.Getenv("HOME")
  	if home != "" {
  		return filepath.Join(home, ".local", "share", "buildkit")
  	}
  	return Root
  }
docs: // UserRoot typically returns /home/$USER/.local/share/buildkit