Do not set userspec if no such option passed (also fix for busybox)

run.sh

@@ -32,17 +32,19 @@ setup () {
 
 runcmd () {
   echo -e '-- \e[1;32mOK\e[0m -- running chroot'
-  userspec="--userspec=$1"
+  userspec="$1"
   chrdir="$2"
   shift 2
   chrcmd=("$@")
-  chroot "$userspec" "$chrdir" "${chrcmd[@]}"
+  [[ "$userspec" != "" ]] && \
+    chroot "--userspec=$userspec" "$chrdir" "${chrcmd[@]}" || \
+    chroot "$chrdir" "${chrcmd[@]}"
 }
 
 main () {
   [[ "$1" == "--help" || "$1" == "" ]] && help && exit 0
 
-  userspec="$(id -u):$(id -g)"
+  userspec=""
 
   while [[ "$1" == -* ]]
   do