Fis systemd support on debian system version 13 (trixie) or newer.

update_clamav-unofficial-sigs.sh

@@ -182,9 +182,11 @@ fi
 
 # - Is this a systemd system?
 # -
-if [[ "X`which systemd`" = "X" ]]; then
-   systemd_exists=false
-else
+systemd_exists=false
+systemd=$(which systemd)
+systemctl=$(which systemctl)
+
+if [[ -n "$systemd" ]] || [[ -n "$systemctl" ]] ; then
    systemd_exists=true
 fi