Replace pkexec with terminal output (with TERMINAL environment var)

ArchUpdater:use terminal thanks to `TERMINAL` environment variable
README: Add explanation for said environment var

Modules/Bar/Widgets/ArchUpdater.qml

@@ -64,6 +64,5 @@ NIconButton {
 
     PanelService.getPanel("archUpdaterPanel").toggle(screen, this)
     ArchUpdaterService.doPoll()
-    ArchUpdaterService.doAurPoll()
   }
-}
+}