HTTPS missing in multilanguage/multidomain setup when rebuilding from Drush or cron

This can actually be solved easily by implementing the following hook.

/**
 * Implements hook_xmlsitemap_context_url_options_alter().
 */
function MYMODULE_xmlsitemap_context_url_options_alter(array &$options, array $context) {
  $options['https'] = TRUE;
}

But to let this be environment-specific I think it's actually better to be able to set that in settings(.local).php.

Set $settings['xmlsitemap_url_options_https'] = TRUE; to enforce HTTPS for all languages.
Set $settings['xmlsitemap_url_options_https']['es'] = TRUE; to enforce HTTPS only for Spanish links.