message: go: Store toolkit.
diff: @@ -621,6 +621,7 @@ class Executable(drake.Node):
  :param build_host: Build for host architecture and OS.
  :type build_host: bool
  """
  + self.__toolkit = toolkit
  target = target or '%s%s' % (source.name().without_last_extension(),
  self.__toolkit.exec_ext(build_host))
  super().__init__(target)
  