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
run_date,
            expected_start_date,
            expected_end_date,
            expected_duration,
            expected_state,
            expected_try_number,
            expected_task_reschedule_count,
        ):
            with freeze_time(run_date):
                try:
                    ti.run()
                except AirflowException:
                    if not fail:
                        raise
            ti.refresh_from_db()
            self.assertEqual(ti.state, expected_state)
            self.assertEqual(ti._try_number, expected_try_number)
            self.assertEqual(ti.try_number, expected_try_number + 1)
            self.assertEqual(ti.start_date, expected_start_date)
            self.assertEqual(ti.end_date, expected_end_date)
            self.assertEqual(ti.duration, expected_duration)
            trs = TaskReschedule.find_for_task_instance(ti)  
            self.assertEqual(len(trs), expected_task_reschedule_count)
        date1 = timezone.utcnow()
        date2 = date1 + datetime.timedelta(minutes=1)
        date3 = date2 + datetime.timedelta(minutes=1)
        date4 = date3 + datetime.timedelta(minutes=1)