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
"""
        Test that tasks properly take start/end dates from DAGs
        """
        dag = DAG('dag', start_date=DEFAULT_DATE, end_date=DEFAULT_DATE + datetime.timedelta(days=10))
        op1 = DummyOperator(task_id='op_1', owner='test')
        self.assertTrue(op1.start_date is None and op1.end_date is None)
        
        dag.add_task(op1)
        self.assertTrue(op1.start_date == dag.start_date and op1.end_date == dag.end_date)
        op2 = DummyOperator(
            task_id='op_2',
            owner='test',
            start_date=DEFAULT_DATE - datetime.timedelta(days=1),
            end_date=DEFAULT_DATE + datetime.timedelta(days=11),
        )
        
        dag.add_task(op2)
        self.assertTrue(op2.start_date == dag.start_date and op2.end_date == dag.end_date)
        op3 = DummyOperator(
            task_id='op_3',
            owner='test',
            start_date=DEFAULT_DATE + datetime.timedelta(days=1),
            end_date=DEFAULT_DATE + datetime.timedelta(days=9),
        )
        
        dag.add_task(op3)
        self.assertTrue(op3.start_date == DEFAULT_DATE + datetime.timedelta(days=1))
        self.assertTrue(op3.end_date == DEFAULT_DATE + datetime.timedelta(days=9))
    def test_timezone_awareness(self):