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
done=done,
            flag_upstream_failed=flag_upstream_failed,
        )
        completed = all(dep.passed for dep in dep_results)
        self.assertEqual(completed, expect_completed)
        self.assertEqual(ti.state, expect_state)
    def test_respects_prev_dagrun_dep(self):
        with DAG(dag_id='test_dag'):
            task = DummyOperator(task_id='task', start_date=DEFAULT_DATE)
        ti = TI(task, DEFAULT_DATE)
        failing_status = [TIDepStatus('test fail status name', False, 'test fail reason')]
        passing_status = [TIDepStatus('test pass status name', True, 'test passing reason')]
        with patch(
            'airflow.ti_deps.deps.prev_dagrun_dep.PrevDagrunDep.get_dep_statuses', return_value=failing_status
        ):
            self.assertFalse(ti.are_dependencies_met())
        with patch(
            'airflow.ti_deps.deps.prev_dagrun_dep.PrevDagrunDep.get_dep_statuses', return_value=passing_status
        ):
            self.assertTrue(ti.are_dependencies_met())
    @parameterized.expand(
        [
            (State.SUCCESS, True),
            (State.SKIPPED, True),
            (State.RUNNING, False),
            (State.FAILED, False),
            (State.NONE, False),
        ]
    )
    def test_are_dependents_done(self, downstream_ti_state, expected_are_dependents_done):