UpdateInfo: Ignore multiple update check requests

The existing code would kill the currently running process, but

- It's unclear why it should do that. If the user wants to cancel the
  currently running one, they can do that in any case.
- The existing code killed the current maintenancetool, but did not
  cancel the task in the progress manager.

Task-number: QTCREATORBUG-28236
Change-Id: I06c2d6b733d637e38cae120dc1a375f7952bfa85
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/plugins/updateinfo/updateinfoplugin.cpp

@@ -140,7 +140,8 @@ void UpdateInfoPlugin::doAutoCheckForUpdates()
 
 void UpdateInfoPlugin::startCheckForUpdates()
 {
-    stopCheckForUpdates();
+    if (d->m_maintenanceToolProcess)
+        return; // do not trigger while update task is already running
 
     QFutureInterface<void> futureIf;
     FutureProgress *futureProgress