mainloop: Ensure we reset running to True when rerun

Otherwise we pretty much exit immediately...

src/ostbuild/pyostbuild/mainloop.py

@@ -85,5 +85,6 @@ class Mainloop(object):
                 self._timeouts[i] = (remaining_ms, callback)
 
     def run(self):
+        self._running = True
         while self._running:
             self.run_once()