[ruby/timeout] Only run timeout_after hook on fiber scheduler if scheduler exists

https://github.com/ruby/timeout/commit/4893cde0ed

lib/timeout.rb

@@ -85,7 +85,7 @@ module Timeout
 
     message ||= "execution expired".freeze
 
-    if (scheduler = Fiber.current_scheduler)&.respond_to?(:timeout_after)
+    if Fiber.respond_to?(:current_scheduler) && (scheduler = Fiber.current_scheduler)&.respond_to?(:timeout_after)
       return scheduler.timeout_after(sec, klass || Error, message, &block)
     end