connect(let_value) is not constrained enough

[glebov-andrey]

Hi,

There appears to be an issue with let_value wrapping single-shot (move only) senders, when the resulting sender is passed to spawn_detached.

The following code doesn't compile:

unifex::any_sender_of<> s = unifex::just();
auto a1 = std::move(s) | unifex::let_value([]() { return unifex::just(); });
auto scope = unifex::v2::async_scope{};
unifex::spawn_detached(std::move(a1), scope);

As far as I understand, the problem is that let_value's connect function is not constrained on sender_to<member_t<Sender, Predecessor>, predecessor_receiver<...>>.
So when async_scope checks requires sender_to<const Sender&, remove_cvref_t<Receiver>> it tries to instantiate let_value's connect which of course fails inside operation on connect_result_t<const Sender &, predecessor_receiver>.