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
super().__init__()
        self.dag_id = task.dag_id
        self.task_id = task.task_id
        self.task = task
        self.refresh_from_task(task)
        self._log = logging.getLogger("airflow.task")
        
        if execution_date and not timezone.is_localized(execution_date):
            self.log.warning(
                "execution date %s has no timezone information. Using default from dag or system",
                execution_date,
            )
            if self.task.has_dag():
                execution_date = timezone.make_aware(execution_date, self.task.dag.timezone)
            else:
                execution_date = timezone.make_aware(execution_date)
            execution_date = timezone.convert_to_utc(execution_date)
        self.execution_date = execution_date
        self.try_number = 0
        self.unixname = getpass.getuser()
        if state: