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
dag = models.DAG(dag_id='test_check_and_change_state_before_execution')
        task = DummyOperator(task_id='task', dag=dag, start_date=DEFAULT_DATE)
        ti = TI(task=task, execution_date=timezone.utcnow())
        self.assertEqual(ti._try_number, 0)
        self.assertTrue(ti.check_and_change_state_before_execution())
        
        self.assertEqual(ti.state, State.RUNNING)
        self.assertEqual(ti._try_number, 1)
    def test_check_and_change_state_before_execution_dep_not_met(self):
        dag = models.DAG(dag_id='test_check_and_change_state_before_execution')
        task = DummyOperator(task_id='task', dag=dag, start_date=DEFAULT_DATE)
        task2 = DummyOperator(task_id='task2', dag=dag, start_date=DEFAULT_DATE)
        task >> task2
        ti = TI(task=task2, execution_date=timezone.utcnow())
        self.assertFalse(ti.check_and_change_state_before_execution())
    def test_try_number(self):
        """
        Test the try_number accessor behaves in various running states
        """
        dag = models.DAG(dag_id='test_check_and_change_state_before_execution')
        task = DummyOperator(task_id='task', dag=dag, start_date=DEFAULT_DATE)
        ti = TI(task=task, execution_date=timezone.utcnow())
        self.assertEqual(1, ti.try_number)
        ti.try_number = 2
        ti.state = State.RUNNING
        self.assertEqual(2, ti.try_number)
        ti.state = State.SUCCESS
        self.assertEqual(3, ti.try_number)
    def test_get_num_running_task_instances(self):