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
"""
        Based on this instance's try_number, this will calculate
        the number of previously attempted tries, defaulting to 0.
        """
        
        
        
        
        
        return self._try_number
    @property
    def next_try_number(self):
        """Setting Next Try Number"""
        return self._try_number + 1
    def command_as_list(  
        self,
        mark_success=False,
        ignore_all_deps=False,
        ignore_task_deps=False,
        ignore_depends_on_past=False,
        ignore_ti_state=False,
        local=False,
        pickle_id=None,
        raw=False,
        job_id=None,
        pool=None,
        cfg_path=None,
    ):
        """
        Returns a command that can be executed anywhere where airflow is