GENERATE_SOURCEMAP=false no longer works for CSS

[silverwind]

It seems since version 2, GENERATE_SOURCEMAP no longer prevents the generation of CSS sourcemaps. Steps to reproduce:

npx create-react-app app && cd app && GENERATE_SOURCEMAP=false yarn build

I see the following files generated:

├── css
│   ├── main.c1872244.chunk.css
│   └── main.c1872244.chunk.css.map
├── js
│   ├── 1.40088a58.chunk.js
│   ├── main.84ef4f68.chunk.js
│   └── runtime~main.229c360f.js

[Timer]

Fixed in #5246