Fix world age issue with custom streams for Distributed workers (#42481)

If connect(::CustomClusterManager, ...) returns a custom transport
stream, use of that stream by the task in start_gc_msgs_task() may fail
due to the task executing in an old world age. Add an invokelatest() to
prevent this problem.

(cherry picked from commit a05bcb2)

stdlib/Distributed/src/remotecall.jl

@@ -251,7 +251,10 @@ const any_gc_flag = Condition()
 function start_gc_msgs_task()
     @async while true
         wait(any_gc_flag)
-        flush_gc_msgs()
+        # Use invokelatest() so that custom message transport streams
+        # for workers can be defined in a newer world age than the Task
+        # which runs the loop here.
+        invokelatest(flush_gc_msgs) # handles throws internally
     end
 end