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
task2 = DummyOperator(
            task_id="test_handle_failure_on_retry",
            on_failure_callback=mock_on_failure_2,
            on_retry_callback=mock_on_retry_2,
            retries=1,
            dag=dag,
        )
        ti2 = TI(task=task2, execution_date=start_date)
        ti2.state = State.FAILED
        ti2.handle_failure("test retry handling")
        mock_on_failure_2.assert_not_called()
        context_arg_2 = mock_on_retry_2.call_args[0][0]
        assert context_arg_2 and "task_instance" in context_arg_2
        
        mock_on_failure_3 = mock.MagicMock()
        mock_on_retry_3 = mock.MagicMock()
        task3 = DummyOperator(
            task_id="test_handle_failure_on_force_fail",
            on_failure_callback=mock_on_failure_3,
            on_retry_callback=mock_on_retry_3,
            retries=1,
            dag=dag,
        )
        ti3 = TI(task=task3, execution_date=start_date)
        ti3.state = State.FAILED
        ti3.handle_failure("test force_fail handling", force_fail=True)
        context_arg_3 = mock_on_failure_3.call_args[0][0]
        assert context_arg_3 and "task_instance" in context_arg_3
        mock_on_retry_3.assert_not_called()
    def test_does_not_retry_on_airflow_fail_exception(self):