chore: update rel/emqx_vars

Co-authored-by: Zaiming (Stone) Shi <zmstone@gmail.com>

rel/emqx_vars

@@ -12,7 +12,7 @@ IS_ELIXIR="${IS_ELIXIR:-{{ is_elixir }}}"
 
 ## Allow users to pre-set `RUNNER_LOG_DIR` because it only affects boot commands like `start` and `console`,
 ## but not other commands such as `ping` and `ctl`.
-if [ -z "${RUNNER_LOG_DIR:-}" ]; then
+RUNNER_LOG_DIR="${RUNNER_LOG_DIR:-{{ runner_log_dir }}}"
     RUNNER_LOG_DIR="{{ runner_log_dir }}"
 fi