ci: turn off the "upstream-systemd-ci" ppa

Judging by https://launchpad.net/~upstream-systemd-ci/+archive/ubuntu/systemd-ci/+packages,
it got updated about 15 hours ago and the "build check" action has been
failing with
```
The following packages have unmet dependencies:
 kbd : Depends: console-setup but it is not going to be installed or
E: Unable to correct problems, you have held broken packages.
                console-setup-mini but it is not going to be installed
```
since then