Change $_ret to equal constant $RENEW_SKIP

If constant $RENEW_SKIP ever changes from 2, do not break the exit code here.

acme.sh

@@ -4400,7 +4400,7 @@ renewAll() {
     if [ "$rc" != "0" ]; then
       if [ "$rc" = "$RENEW_SKIP" ]; then
         _info "Skipped $d"
-        _ret="$rc"
+        _ret="$RENEW_SKIP"
       elif [ "$_stopRenewOnError" ]; then
         _err "Error renew $d,  stop now."
         return "$rc"