never run next in firebase function in dev mode

- related to vercel#8893

examples/with-firebase-hosting/firebaseFunctions.js

@@ -2,11 +2,10 @@ const { join } = require('path')
 const { https } = require('firebase-functions')
 const { default: next } = require('next')
 
-const isDev = process.env.NODE_ENV !== 'production'
 const nextjsDistDir = join('src', require('./src/next.config.js').distDir)
 
 const nextjsServer = next({
-  dev: isDev,
+  dev: false,
   conf: {
     distDir: nextjsDistDir,
   },