Fix build issue

Build did not work on latest version of "create astro".

```
  "dependencies": {
    "astro": "^2.0.2"
  },
  "devDependencies": {
    "@matthewp/astro-fastify": "^2.0.3"
  }
```

Issue origin: withastro/astro#5056

lib/index.js

@@ -117,7 +117,7 @@ export default function(options) {
           }
         }
       },
-      'astro:build:start'({ buildConfig }) {
+      'config.build'({ buildConfig }) {
         args.clientRelative = relative(fileURLToPath(buildConfig.server), fileURLToPath(buildConfig.client));
       }
     }