Fix firstRun special case so we don't continue to check zones.

Also don't leave up zone checking progress reporter after it has
been cancelled.

Fixes https://gitlab.com/kicad/code/kicad/issues/6077

pcbnew/dialogs/dialog_cleanup_tracks_and_vias.cpp

@@ -125,6 +125,7 @@ void DIALOG_CLEANUP_TRACKS_AND_VIAS::doCleanup( bool aDryRun )
 
         m_changesTreeModel->SetProvider( nullptr );
         m_items.clear();
+        m_firstRun = false;
     }
 
     // Old model has to be refreshed, GAL normally does not keep updating it