chore: rename DB_ROLE to ROLE

bin/emqx

@@ -986,7 +986,7 @@ if [[ "$IS_BOOT_COMMAND" == 'yes' && "$(get_boot_config 'node.db_backend')" == "
     if ! (echo -e "$COMPATIBILITY_INFO" | $GREP -q 'MNESIA_OK'); then
       logwarn "DB Backend is RLOG, but an incompatible OTP version has been detected. Falling back to using Mnesia DB backend."
       export EMQX_NODE__DB_BACKEND=mnesia
-      export EMQX_NODE__DB_ROLE=core
+      export EMQX_NODE__ROLE=core
     fi
 fi