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
"""
        iso = execution_date.isoformat()
        cmd = ["airflow", "tasks", "run", dag_id, task_id, iso]
        if mark_success:
            cmd.extend(["--mark-success"])
        if pickle_id:
            cmd.extend(["--pickle", str(pickle_id)])
        if job_id:
            cmd.extend(["--job-id", str(job_id)])
        if ignore_all_deps:
            cmd.extend(["--ignore-all-dependencies"])
        if ignore_task_deps:
            cmd.extend(["--ignore-dependencies"])
        if ignore_depends_on_past:
            cmd.extend(["--ignore-depends-on-past"])
        if ignore_ti_state:
            cmd.extend(["--force"])
        if local:
            cmd.extend(["--local"])
        if pool:
            cmd.extend(["--pool", pool])
        if raw:
            cmd.extend(["--raw"])
        if file_path:
            cmd.extend(["--subdir", file_path])
        if cfg_path:
            cmd.extend(["--cfg-path", cfg_path])
        return cmd
    @property
    def log_filepath(self):