`scheduler` concept is missing a requirement regarding domains

[ericniebler]

get_domain(sch) returns the domain tag of sch's associated execution context. so too does get_domain(get_env(schedule(sch))). nothing is currently requiring them to have the same type.

Proposed resolution

Change the scheduler concept as follows:

namespace std::execution {
  template<class Env>
    using domain-of-t =                       // exposition only
      decltype(auto(query-with-default(get_domain, declval<Env>(), default_domain())));

  template<class Sch>
    concept scheduler =
      derived_from<typename remove_cvref_t<Sch>::scheduler_concept, scheduler_t> &&
      queryable<Sch> &&
      requires(Sch&& sch) {
        { schedule(std::forward<Sch>(sch)) } -> sender;
        { auto(get_completion_scheduler<set_value_t>(
            get_env(schedule(std::forward<Sch>(sch))))) }
              -> same_as<remove_cvref_t<Sch>>;
      } &&
      same_as<domain-of-t<Sch>, domain-of-t<env_of_t<decltype(schedule(declval<Sch>()))>>> &&
      equality_comparable<remove_cvref_t<Sch>> &&
      copyable<remove_cvref_t<Sch>>;
}