Undo part of the change to task's await_transform

PR #628 means that asking "is `unifex::await_transform` invocable on
this awaited thing?" always returns "yes" so this change reverts to
asking "has this awaited thing customized `await_transform` or is it
naturally awaitable?" like we used to.

Author: ispeters

include/unifex/task.hpp

@@ -377,10 +377,9 @@ struct _promise final {
         return unifex::await_transform(
             *this,
             with_scheduler_affinity(static_cast<Value&&>(value), this->sched_));
-      } else if constexpr (std::is_invocable_v<
-                               tag_t<unifex::await_transform>,
-                               type&,
-                               Value>) {
+      } else if constexpr (
+          tag_invocable<tag_t<unifex::await_transform>, type&, Value> ||
+          detail::_awaitable<Value>) {
         // await_transform has been customized so we can dispatch to the
         // await_transform CPO, then insert a transition back to the correct
         // execution context if necessary.