test: install /etc/default/knot if available as well
    
    The knot.service on Ubuntu Jammy loads an env file which we didn't
    install, causing the service to fail:
    
    ```
    knot.service: Will spawn child (service_enter_start_pre): /usr/sbin/knotc
    knot.service: Failed to load environment files: No such file or directory
    knot.service: Failed to run 'start-pre' task: No such file or directory
    knot.service: Failed with result 'resources'.
    knot.service: Service will not restart (restart setting)
    ```