hlwm: use shell substitution instead of cut

xdg-config-dir/herbstluftwm/autostart

@@ -166,6 +166,7 @@ hc unlock
 
 hc detect_monitors
 
-for m in $(hc list_monitors | cut -d : -f 1); do
+hc list_monitors | while read -r line; do
+	m=${line%%:*}
 	sh ~/.config/herbstluftwm/herbstpanel.sh "$m" &
 done