Ensure that platform from group selection checks broadcast manager

for updates to task config.
Ensure that 255 callback gives a sensible platform in warning messages.

cylc/flow/subprocpool.py

@@ -557,12 +557,12 @@ def _run_callback(callback, args_=None):
                 # Backup, get a platform name from the config:
                 for arg in callback_args:
                     if isinstance(arg, TaskProxy):
-                        platform_name = arg.tdef.rtconfig['platform']
+                        platform_name = arg.platform['name']
                     elif (
                         isinstance(arg, list)
                         and isinstance(arg[0], TaskProxy)
                     ):
-                        platform_name = arg[0].tdef.rtconfig['platform']
+                        platform_name = arg[0].platform['name']
 
         if cls.ssh_255_fail(ctx) or cls.rsync_255_fail(ctx, platform) is True:
             # Job log retrieval passes a special object as a command key

cylc/flow/task_job_mgr.py

@@ -306,10 +306,12 @@ def submit_task_jobs(self, workflow, itasks, curve_auth,
 
                     # Get another platform, if task config platform is a group
                     use_next_platform_in_group = False
-                    if itask.tdef.rtconfig['platform']:
+                    bc_mgr = self.task_events_mgr.broadcast_mgr
+                    rtconf = bc_mgr.get_updated_rtconfig(itask)
+                    if rtconf['platform']:
                         try:
                             platform = get_platform(
-                                itask.tdef.rtconfig['platform'],
+                                rtconf['platform'],
                                 bad_hosts=self.bad_hosts
                             )
                         except PlatformLookupError: