[bench/server] pass corrct command to subprocess

bench/server.py

@@ -58,7 +58,7 @@ class Server(subprocess.Popen):
 
         server_env = prepare_env(env or {})
 
-        super().__init__(cmd.split(),
+        super().__init__(self.cmd.split(),
                          stdout=self.fout,
                          stderr=self.ferr,
                          env=server_env,