Should stdexec::spawn accept an lvalue scope token?

[gamecentric]

Hi,
it seems that spawn accepts only rvalue scope tokens:

counting_scope scope;
spawn(just(), scope.get_token()); // ok

auto token = scope.get_token();
spawn(just(), token); // error: counting_scope::token& does not satisfy the scope_token concept
spawn(just(), std::move(token)); // ok

Scope tokens are required to copiable and all token types in the C++ standard are usually expected to be cheaply copiable. In the specific case of counting_scope::token it's just a pointer to the scope. Thus the restriction seems quite unnatural.

[ericniebler]

fixed by #1966