Fixes dynamic chunks preload (#4992)

based on [this](https://spectrum.chat/thread/0d24d6cc-96f9-46f0-aa86-515c1b1caac9?m=MTUzNDc2MjA3MzkyMQ==) conversation

server/document.js

@@ -67,8 +67,8 @@ export class Head extends Component {
   getPreloadDynamicChunks () {
     const { dynamicImports, assetPrefix } = this.context._documentProps
     return dynamicImports.map((bundle) => {
-      return <script
-        async
+      return <link
+        rel='preload'
         key={bundle.file}
         src={`${assetPrefix}/_next/${bundle.file}`}
         as='script'