Add non-free to main sources.list

The main file is the only one handled by the Ansible role, so don't add
this to another file that would have to then be managed too.

install.sh

@@ -367,7 +367,7 @@ echo "done."
 
 echo -n "Adding non-free repository (firmware, etc.)... "
 mkdir -p ${target}/etc/apt/sources.list.d/ >&2
-echo "deb ${debmirror} ${debrelease} contrib non-free" | tee -a ${target}/etc/apt/sources.list.d/non-free.list >&2
+echo "deb ${debmirror} ${debrelease} contrib non-free" | tee -a ${target}/etc/apt/sources.list >&2
 chroot ${target} apt update >&2
 echo "done."