DEV: Collect css assets from webpack build (#25087)

fb95ab8e00 started depending on webpack-bundled CSS, but css files weren't being collected by Sprockets or uploaded to S3.

lib/ember_cli.rb

@@ -9,7 +9,7 @@ class EmberCli < ActiveSupport::CurrentAttributes
   end
 
   def self.assets
-    cache[:assets] ||= Dir.glob("**/*.{js,map,txt}", base: "#{dist_dir}/assets")
+    cache[:assets] ||= Dir.glob("**/*.{js,map,txt,css}", base: "#{dist_dir}/assets")
   end
 
   def self.script_chunks