bug: Flet build template ignores ENV VARs #6393

if (defaultTargetPlatform == TargetPlatform.windows) {
      // use TCP on Windows
      var tcpPort = await getUnusedPort();
      pageUrl = "tcp://localhost:$tcpPort";
      environmentVariables["FLET_SERVER_PORT"] = tcpPort.toString();
    } else {
      // use UDS on other platforms
      pageUrl = "flet_$pid.sock";
      environmentVariables["FLET_SERVER_UDS_PATH"] = pageUrl;
    }

ServerSocket outSocketServer;
  String socketAddr = "";
  StringBuffer pythonOut = StringBuffer();

  if (defaultTargetPlatform == TargetPlatform.windows) {
    var tcpAddr = "127.0.0.1";
    outSocketServer = await ServerSocket.bind(tcpAddr, 0);
    debugPrint(
        'Python output TCP Server is listening on port ${outSocketServer.port}');
    socketAddr = "$tcpAddr:${outSocketServer.port}";
  } else {
    socketAddr = "stdout_$pid.sock";
    if (await File(socketAddr).exists()) {
      await File(socketAddr).delete();
    }
    outSocketServer = await ServerSocket.bind(
        InternetAddress(socketAddr, type: InternetAddressType.unix), 0);
    debugPrint('Python output Socket Server is listening on $socketAddr');
  }

  environmentVariables["FLET_PYTHON_CALLBACK_SOCKET_ADDR"] = socketAddr;