Build tools: After [48277], ignore CSS files in...

Build tools: After [48277], ignore CSS files in `src/wp-admin/css/colors/modern` when building WordPress to `src`.

See #50504.
Built from https://develop.svn.wordpress.org/trunk@48286


git-svn-id: https://core.svn.wordpress.org/trunk@48055 1a063a9b-81f0-0310-95a4-ce76da25c4cd