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


airflow/models/taskinstance.py
"""Key used to identify task instance."""
    dag_id: str
    task_id: str
    execution_date: datetime
    try_number: int
    @property
    def primary(self) -> Tuple[str, str, datetime]:
        """Return task instance primary key part of the key"""
        return self.dag_id, self.task_id, self.execution_date
    @property
    def reduced(self) -> 'TaskInstanceKey':