MDEV-25690 Plugins can't execute sql statements with the Galera enabled.

For the plugin thread during the server start the wsrep_on should be 0.

Author: Alexey Botchkov

sql/sql_prepare.cc

@@ -6082,6 +6082,7 @@ extern "C" int execute_sql_command(const char *command,
  new_thd->store_globals();
  new_thd->security_ctx->skip_grants();
  new_thd->query_cache_is_applicable= 0;
+ new_thd->variables.wsrep_on= 0;
  bzero((char*) &new_thd->net, sizeof(new_thd->net));
  thd= new_thd;
  }