How can I change the temporary file directory path without the UI?
Drush

drush config:set system.file path.temporary /tmp -y

settings.php

If done through settings.php it will no longer be changeable via either the interface or Drush. 

Drupal 8.8+

See the change record.

$settings['file_temp_path'] = '/tmp';


Drupal 8.7 and below

$config['system.file']['path']['temporary'] = '/tmp';