fix border radius of loading spinner

Author: akowalska622

src/platform/packages/shared/kbn-unified-tabs/src/components/tabs_visual_glue_to_app_container/tab_with_background.tsx

@@ -41,6 +41,7 @@ export const TabWithBackground = React.forwardRef<HTMLDivElement, TabWithBackgro
           margin: ${euiTheme.size.xs};
           margin-bottom: 0;
           padding-bottom: ${euiTheme.size.xs};
+          overflow: hidden;
 
           ${isSelected
             ? `