fix(manifest): run only on build

packages/manifest/index.js

@@ -1,10 +1,18 @@
 const hash = require('hash-sum')
+const debug = require('debug')('nuxt:pwa:manifest')
 
 const fixUrl = url => url.replace(/\/\//g, '/').replace(':/', '://')
 const isUrl = url => url.indexOf('http') === 0 || url.indexOf('//') === 0
 const find = (arr, key, val) => arr.find(obj => val ? obj[key] === val : obj[key])
 
 module.exports = function nuxtManifest (options) {
+  this.nuxt.plugin('build', builder => {
+    debug('Adding manifest')
+    addManifest.call(this, options)
+  })
+}
+
+function addManifest (options) {
   // routerBase and publicPath
   const routerBase = this.options.router.base
   let publicPath = fixUrl(`${routerBase}/${this.options.build.publicPath}`)