fix(boot): allow user defined data_dir

rel/emqx_vars

@@ -22,5 +22,6 @@ export EMQX_DESCRIPTION='{{ emqx_description }}'
 ## computed vars
 REL_NAME="emqx"
 ERTS_PATH="$RUNNER_ROOT_DIR/erts-$ERTS_VSN/bin"
+RUNNER_DATA_DIR="${EMQX_NODE__DATA_DIR:-$RUNNER_DATA_DIR}"
 
 ## updated vars here