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
self.assertEqual(ti._try_number, 1)
        self.assertEqual(ti.try_number, 2)
        
        run_with_error(ti)
        self.assertEqual(ti.state, State.FAILED)
        self.assertEqual(ti._try_number, 2)
        self.assertEqual(ti.try_number, 3)
        
        
        dag.clear()
        
        run_with_error(ti)
        self.assertEqual(ti.state, State.UP_FOR_RETRY)
        self.assertEqual(ti._try_number, 3)
        self.assertEqual(ti.try_number, 4)
        
        run_with_error(ti)
        ti.refresh_from_db()
        self.assertEqual(ti.state, State.FAILED)
        self.assertEqual(ti._try_number, 4)
        self.assertEqual(ti.try_number, 5)
        self.assertEqual(RenderedTaskInstanceFields.get_templated_fields(ti), expected_rendered_ti_fields)
    def test_next_retry_datetime(self):
        delay = datetime.timedelta(seconds=30)
        max_delay = datetime.timedelta(minutes=60)
        dag = models.DAG(dag_id='fail_dag')
        task = BashOperator(
            task_id='task_with_exp_backoff_and_max_delay',
            bash_command='exit 1',
            retries=3,