Simplify copying files in Active Storage migration

We can use the `ActiveStorage::Blob` class to find where the file is
supposed to be stored instead of manually setting the path. This is also
more robust because it works with Active Storage configurations which
don't store files in the default folder.

lib/tasks/active_storage.rake

@@ -61,16 +61,11 @@ namespace :active_storage do
     end
 
     ActiveStorage::Attachment.find_each do |attachment|
+      dest = ActiveStorage::Blob.service.path_for(attachment.blob.key)
       name = attachment.name
       source = attachment.record.send(name).path
 
-      dest_dir = File.join(
-        "storage",
-        attachment.blob.key.first(2),
-        attachment.blob.key.first(4).last(2))
-      dest = File.join(dest_dir, attachment.blob.key)
-
-      FileUtils.mkdir_p(dest_dir)
+      FileUtils.mkdir_p(File.dirname(dest))
       logger.info "Copying #{source} to #{dest}"
       FileUtils.cp(source, dest)
     end