bin: don't start a shell with cdtemp prune

.local/bin/cdtemp

@@ -12,6 +12,7 @@ if [ "$cmd" = "last" ]; then
 elif [ "$cmd" = "prune" ]; then
   echo "Deleting empty directories"
   find "$TEMPSPACE" -mindepth 1 -maxdepth 1 -type d -empty -delete -print
+  exit
 else
   dir="$TEMPSPACE/$(date +%s)"
   mkdir "$dir"