Fix global SCSS on Node.js <= 14 (#50185)

Fixes #45052. See the comments and related issue thread.

packages/next/src/build/webpack/config/blocks/css/index.ts

@@ -158,7 +158,17 @@ export const css = curry(async function css(
         // Source maps are required so that `resolve-url-loader` can locate
         // files original to their source directory.
         sourceMap: true,
-        sassOptions,
+        sassOptions: {
+          // The "fibers" option is not needed for Node.js 16+, but it's causing
+          // problems for Node.js <= 14 users as you'll have to manually install
+          // the `fibers` package:
+          // https://github.com/webpack-contrib/sass-loader#:~:text=We%20automatically%20inject%20the%20fibers%20package
+          // https://github.com/vercel/next.js/issues/45052
+          // Since it's optional and not required, we'll disable it by default
+          // to avoid the confusion.
+          fibers: false,
+          ...sassOptions,
+        },
         additionalData: sassPrependData || sassAdditionalData,
       },
     },