feat: add Next.js runtime 4.39.1 behind a feature flag (#1227)

Author: Rob Stanford

site/plugins.json

@@ -611,6 +611,10 @@
     "repo": "https://github.com/netlify/next-runtime",
     "version": "4.39.0",
     "compatibility": [
+      {
+        "version": "4.39.1",
+        "featureFlag": "build_plugins_use_prerelease"
+      },
       {
         "version": "4.39.0",
         "migrationGuide": "https://ntl.fyi/next-plugin-migration"