Creates a Drupal Ajax 'update_build_id' command.
This command updates the value of a hidden form_build_id input element on a form. It requires the form passed in to have keys for both the old build ID in #build_id_old and the new build ID in #build_id.
The primary use case for this Ajax command is to serve a new build ID to a form served from the cache to an anonymous user, preventing one anonymous user from accessing the form state of another anonymous users on Ajax enabled forms.
$form: The form array representing the form whose build ID should be updated.
function ajax_command_update_build_id($form) {
return array(
'command' => 'updateBuildId',
'old' => $form['#build_id_old'],
'new' => $form['#build_id'],
);
}