nsxiv exec: use foot instead of hardcoded

.config/nsxiv/exec/key-handler

@@ -5,7 +5,7 @@ while read file; do
 	case "$1" in
 	"c") xclip -sel clipboard "$file" -t image/png ;; # copy image
 	"d") dragon-drop "$file" & ;; # drag and drop
-	"o") urxvt -e nnn "$file" & ;; # open in nnn
+	"o") $TERMINAL -e nnn "$file" & ;; # open in nnn
 	"p") printf '%s' "$file" | xclip -sel clipboard ;; # copy path
 	"C-p") printf '%s' "${file##*/}" | xclip -sel clipboard ;; # copy basename
 	esac