scheduler: write contact file before configure

* Write the contact file earlier to minimise the window of opportunity
  for two schedulers to start up for the same workflow.

cylc/flow/scheduler.py

@@ -699,8 +699,8 @@ async def start(self):
             self.server.thread.start()
             barrier.wait()
 
-            await self.configure()
             self._configure_contact()
+            await self.configure()
         except (KeyboardInterrupt, asyncio.CancelledError, Exception) as exc:
             await self.handle_exception(exc)