For xcoms pushed from different tasks, having the same execution_date and in the same Dag, the result of the xcom_pull with key set as None is not deterministic in its order

**Apache Airflow version**: 2.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**:
When tasks in the same dag push Xcoms, the result of the xcom_pull comes in any order. The order is not deterministic.

**What you expected to happen**:

I expected the order to be deterministic.

**How to reproduce it**:

Run the provided example_xcom dag in the repository multiple times


tests/models/test_taskinstance.py
start_date = timezone.datetime(2016, 6, 1)
        dag = models.DAG(dag_id="test_handle_failure", schedule_interval=None, start_date=start_date)
        mock_on_failure_1 = mock.MagicMock()
        mock_on_retry_1 = mock.MagicMock()
        task1 = DummyOperator(
            task_id="test_handle_failure_on_failure",
            on_failure_callback=mock_on_failure_1,
            on_retry_callback=mock_on_retry_1,
            dag=dag,
        )
        ti1 = TI(task=task1, execution_date=start_date)
        ti1.state = State.FAILED
        ti1.handle_failure("test failure handling")
        context_arg_1 = mock_on_failure_1.call_args[0][0]
        assert context_arg_1 and "task_instance" in context_arg_1
        mock_on_retry_1.assert_not_called()
        mock_on_failure_2 = mock.MagicMock()
        mock_on_retry_2 = mock.MagicMock()