ci: enable systemd repo build testing

This creates an additional `*-systemd` build job for each CPU arch, so now there are two jobs for each arch:

1) The original build job: build changed aports with pmb systemd config set to default
2) The new '*-systemd' job: build changes aports with pmb systemd config set to 'always'

In the first job, build_changed_aports uses pmb to figure out if it
can build the changed aport(s), so that it can skip any changed aports
that are in extra-repos/systemd (these would trigger a pmb failure if
it tried)

In the second job, kernel packages are skipped since we do not want to
rebuild these, it makes no sense to rebuild them in the context of the
systemd repo when the first build job will handle build testing them.