ci: enable Packit integration

Let's enable the Packit integration and see if it's a viable option for
us. This configuration builds systemd on Fedora (on x86_64, i386, and
aarch64) and runs the unit test suite. To do that, it uses the specfile
from Fedora Rawhide[0] with some minor modifications, thus dropping the
need to have a specfile in the upstream repository.

So far the builds took around 25 minutes each, so speed-wise it's pretty
good. The two remaining supported architectures (s390x and armhfp) are
excluded, for now, since they're emulated and build there takes a really
long time (~4 hours).

[0] https://src.fedoraproject.org/rpms/systemd/