Don't attempt to restart systemd-networkd if systemd is not running
(e.g. installing open5gs inside a chroot).
Fix for:
System has not been booted with systemd as init system (PID 1). Can't operate.
Failed to connect to bus: Host is down
dpkg: error processing package open5gs-upf:amd64 (--configure):
installed open5gs-upf:amd64 package post-installation script subprocess returned error exit status 1
fe11ee161c