receiver/sender concepts require `get_env` but this has a fallback that accepts any type

[lewissbaker]

The receiver and sender concepts both require that the type supports calling get_env(obj) on an object of that type. e.g.

// [exec.recv.concepts]
template<class Rcvr>
    concept receiver =
      derived_from<typename remove_cvref_t<Rcvr>::receiver_concept, receiver_t> &&
      requires(const remove_cvref_t<Rcvr>& rcvr) {
        { get_env(rcvr) } -> queryable;
      } &&
      move_constructible<remove_cvref_t<Rcvr>> &&       // rvalues are movable, and
      constructible_from<remove_cvref_t<Rcvr>, Rcvr>;   // lvalues are copyable

However, the get_env() CPO is defined to have a fall-back implementation that gets invoked if the object doesn't have a .get_env() member function, which just returns an empty environment.

[exec.get.env] says:

For a sub-expression o, execution::get_env(0) is expression-equivalent to:

• MANDATE-NOTHROW(as_const(o).get_env()) if that expression is well-formed.
  Mandates: The type of the expression above satisfies queryable ([exec.queryable]])
• Otherwise, env<>{}.

This means that the constraint that you can call get_env is not going to be very selective as it will work with any type that hasn't defined a non-compliant get_env member-function.
Do we need to have the constraint that get_env is callable here?