test: install /etc/default/knot if available as well

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)
```