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

.github/workflows/ubuntu-build-check.sh

@@ -85,7 +85,7 @@ else
 fi
 
 # PPA with some newer build dependencies (like zstd)
-add-apt-repository -y ppa:upstream-systemd-ci/systemd-ci
+#add-apt-repository -y ppa:upstream-systemd-ci/systemd-ci
 apt-get -y update
 apt-get -y build-dep systemd
 apt-get -y install "${PACKAGES[@]}"