def ruby_install_binstub_path(ruby_layer_path = ".")
    @ruby_install_binstub_path ||=
      if ruby_version
        "#{ruby_layer_path}/#{slug_vendor_ruby}/bin"
      else
        ""
      end
  end
find the ruby install path for its binstubs during build
@return [String] resulting path or empty string if ruby is not vendored