Fix: Correct Usage of CleanWebpackPlugin (#1565)

I was still using an option that no longer exists. Fixed to mimic the original behavior. The option `root` was removed in v2, but wasn't changed during the upgrade in 9804259

It's deleting everything in the styleguidistDir folder which isn't the way it was before.
After this change it only deletes files in build/.

src/scripts/make-webpack-config.ts

@@ -98,7 +98,7 @@ export default function(
 			},
 			plugins: [
 				new CleanWebpackPlugin({
-					root: config.styleguideDir,
+					cleanOnceBeforeBuildPatterns: [`${config.styleguideDir}/build/**/*`],
 					verbose: config.verbose === true,
 				} as any),
 			],