Add smart cache to webhooks ISR example

Author: josephfusco

examples/next/webhooks-isr/.wp-env.json

@@ -1,7 +1,8 @@
 {
-  "phpVersion": "7.4",
+  "phpVersion": "8.0",
   "plugins": [
     "https://github.com/wp-graphql/wp-graphql/releases/latest/download/wp-graphql.zip",
+    "https://github.com/wp-graphql/wp-graphql-smart-cache/releases/download/v2.0.0/wpgraphql-smart-cache.zip",
     "https://downloads.wordpress.org/plugin/code-snippets.3.6.8.zip",
     "../../../plugins/wp-graphql-headless-webhooks"
   ],