asio senders don't work with stdexec::task coroutines

[maxbachmann]

stdexec::task<void> test(asio::steady_timer& timer)
{
    co_await timer.async_wait(asio::as_tuple(exec::asio::use_sender));
}

fails to compile:

stdexec/__detail/__task.hpp:503:37:   required from 'constexpr auto stdexec::task<_Ty, _Env>::__promise::await_transform(_Sender&&) [with _Sender = experimental::execution::asio::detail::use_sender::sender<experimental::execution::asio::detail::completion_token::sender<stdexec::completion_signatures<stdexec::set_value_t(std::tuple<std::error_code>), stdexec::set_error_t(std::__exception_ptr::exception_ptr), stdexec::set_stopped_t()>, asio::async_result<asio::as_tuple_t<experimental::execution::asio::use_sender_t>, void(std::error_code)>::init_wrapper<asio::basic_waitable_timer<std::chrono::_V2::steady_clock>::initiate_async_wait> > >; _Ty = void; _Env = stdexec::env<>]'
  503 |         return STDEXEC::as_awaitable(STDEXEC::affine_on(static_cast<_Sender&&>(__sndr)), *this);
test.cpp:540:69:   required from here
  540 |     co_await timer.async_wait(asio::as_tuple(exec::asio::use_sender));
      |                                                                     ^
stdexec/__detail/__get_completion_signatures.hpp:62:84: error: invalid use of void expression
   62 |     STDEXEC_PP_FRONT(__VA_ARGS__))::template get_completion_signatures<__VA_ARGS__>()
stdexec/__detail/__get_completion_signatures.hpp:66:5: note: in definition of macro 'STDEXEC_CHECKED_COMPLSIGS'
   66 |     __VA_ARGS__, static_cast<__mlist<STDEXEC_PP_EXPAND _ARGS>*>(nullptr))
      |     ^~~~~~~~~~~
stdexec/__detail/__get_completion_signatures.hpp:228:59: note: in expansion of macro 'STDEXEC_GET_COMPLSIGS'
  228 |         return STDEXEC_CHECKED_COMPLSIGS((_Sender, _Env), STDEXEC_GET_COMPLSIGS(_Sender, _Env));
      |                                                           ^~~~~~~~~~~~~~~~~~~~~
test.cpp: In function 'stdexec::task<void> test(asio::steady_timer&)':
test.cpp:540:5: error: no member named 'await_ready' in 'stdexec::{anonymous}::__sexpr<<lambda closure object>stdexec::<lambda()>()>'
  540 |     co_await timer.async_wait(asio::as_tuple(exec::asio::use_sender));
      |     ^~~~~~~~
scons: *** [test.o] Error 1

This works fine with the old exec::task

[maxbachmann]

Probably something similar to #1929