Install new System76 hosted repository for Ubuntu

[jackpot51]

This will replace the PPA with our own repo, to reduce maintenance cost and build times.

Either pop-default-settings must be installed, as on Pop, or the new https://github.com/pop-os/system76-ubuntu-repo

[leviport]

system76-driver is not updating cleanly after adding this testing source:

system76@system76-Lemur-Pro:~$ sudo apt full-upgrade 
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Calculating upgrade... Done
The following packages have been kept back:
  system76-driver
0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.
system76@system76-Lemur-Pro:~$ sudo apt install system76-driver
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 system76-driver : Depends: pop-default-settings but it is not installable or
                            system76-ubuntu-repo but it is not installable
E: Unable to correct problems, you have held broken packages.

I'm not sure whether this is due to the attack or not.

[jackpot51]

@leviport system76-ubuntu-repo may not be released yet

[jackpot51]

I tried to release it on launchpad, hopefully by tomorrow it will be available...

[leviport]

Looks like that did the trick!

After the update, I now see system76-ubuntu-repo.sources in /etc/apt/sources.list.d/. The ubuntu.sources file is still present though. Is that meant to be left behind?