run_loop::schedule() should not have set_stopped_t() in its completion signatures if passed an env with an unstoppable_token

[lewissbaker]

[exec.run.loop.types] p5 specifies that the completion-signatures of the run-loop-sender are always:

  completion_signatures<set_value_t(), set_error_t(exception_ptr), set_stopped_t()>

However, if the environment of the receiver connected to this sender has an unstoppable stop-token then it will never call set_stopped_t. We should conditionally add the set_stopped_t completion-signature only if the environment has a stop-token where a stop is possible.