workspace-indicator: Use existing property

We already track the current workspace index, use that
instead of getting it from the workspace manager again.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/307>

extensions/workspace-indicator/workspaceIndicator.js

@@ -439,7 +439,7 @@ export class WorkspaceIndicator extends PanelMenu.Button {
             return;
 
 
-        let newIndex = global.workspace_manager.get_active_workspace_index() + diff;
+        const newIndex = this._currentWorkspace + diff;
         this._activate(newIndex);
     }
 }