rpm triggers: do nothing if systemd is not running (#5065)

If we are running in a chroot/container/..., we would print a useless warning about
not being able to communicate with systemd. Trying to do daemon-reload is pointless
in those cases, so let's just skip all actions in that case.

The check uses /run/system/system, as recommended by sd_booted(3).

https://bugzilla.redhat.com/show_bug.cgi?id=1411299
(cherry picked from commit 13749f54)