Adapt fai-server to work under trisquel

The sources.list has been extracted from a Trisquel livecd and
a whitespace issue was fixed along the way.

BugLink: https://trisquel.info/en/issues/28007


Authored-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>
Co-Authored-by: Pablo Correa Gómez <ablocorrea@hotmail.com>