workspace: Update bg border radius in constructor

Otherwise non-zero initial value of stateAdjustment will produce
incorrect radius.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1964>

js/ui/workspace.js

@@ -962,6 +962,8 @@ class WorkspaceBackground extends St.Widget {
             });
         this._updateRoundedClipBounds();
 
+        this._updateBorderRadius();
+
         this.connect('destroy', this._onDestroy.bind(this));
     }