Make sure we actually update the local resource roots (#122957)

For #122894

Not sure why this was a no-op but it seems to prevent the webview from correctly loading preloads

src/vs/workbench/contrib/webview/browser/baseWebviewElement.ts

@@ -372,8 +372,11 @@ export abstract class BaseWebview<T extends HTMLElement> extends Disposable {
 		});
 	}
 
-	public set localResourcesRoot(resources: URI[]) {
-		/** no op */
+	public set localResourcesRoot(resources: readonly URI[]) {
+		this.content = {
+			...this.content,
+			options: { ...this.content.options, localResourceRoots: resources }
+		};
 	}
 
 	public set state(state: string | undefined) {