next build doesn't build CSS into output directory.

[chilijung]

Bug report

Describe the bug

When using @zeit/next-css in next.config.js, in dev mode the CSS successfully built into .next/static/style.css but when using next build to build static version of the app will not generate .next/static/style.css in the directory.

To Reproduce

You could reproduce this error in this demo https://github.com/Canner/canner-demo/tree/master/nextjs-ecom-with-firebase

next.config.js is located: https://github.com/Canner/canner-demo/blob/master/nextjs-ecom-with-firebase/functions/next.config.js

Steps to reproduce the behavior, please provide code snippets or a repository:

1. run npm run dev in dev mode css is generated.

2, run ./node_modules/.bin/next build css is NOT generated in output directory.

Expected behavior

When entering next build in output folder it should also generate CSS files.

System information

• OS: [e.g. macOS, Windows]: mac
• Browser (if applies) [e.g. chrome, safari]: chrome
• Version of Next.js: [e.g. 6.0.2]: 6.1.1

Additional context

Add any other context about the problem here.

[timneutkens]

This is unrelated to Next.js. Please file on next-plugins.