feat: add feature flag to v 4.30.4 of next runtime (#1121)

Author: sarahetter

site/plugins.json

@@ -611,6 +611,10 @@
     "repo": "https://github.com/netlify/next-runtime",
     "version": "4.30.3",
     "compatibility": [
+      {
+        "version": "4.30.4",
+        "featureFlag": "build_plugins_use_prerelease"
+      },
       {
         "version": "4.30.3",
         "migrationGuide": "https://ntl.fyi/next-plugin-migration"