Fis error in determin if systemd support is present in case of debian 13 and newer.

install_dehydrated.sh

@@ -1610,10 +1610,10 @@ done
 # -Is systemd supported on this system?
 # -
 systemd_supported=false
-systemd=$(which systemd)
-systemctl=$(which systemctl)
+systemd=\$(which systemd)
+systemctl=\$(which systemctl)
 
-if [[ -n "$systemd" ]] || [[ -n "$systemctl" ]] ; then
+if [[ -n "\$systemd" ]] || [[ -n "\$systemctl" ]] ; then
    systemd_supported=true
 fi