test: fix flaky legacy structuredClone fail (#14522)

Author: sapphi-red

playground/legacy/vite.config-no-polyfills-no-systemjs.js

@@ -18,6 +18,7 @@ export default defineConfig({
   ],
 
   build: {
+    outDir: 'dist/no-polyfills-no-systemjs',
     rollupOptions: {
       input: {
         index: path.resolve(__dirname, 'no-polyfills-no-systemjs.html'),

playground/legacy/vite.config-no-polyfills.js

@@ -17,6 +17,7 @@ export default defineConfig({
   ],
 
   build: {
+    outDir: 'dist/no-polyfills',
     rollupOptions: {
       input: {
         index: path.resolve(__dirname, 'no-polyfills.html'),