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
"""Get the email subject content for exceptions."""
        
        
        use_default = not hasattr(self, 'task')
        exception_html = str(exception).replace('\n', '<br>')
        default_subject = 'Airflow alert: {{ti}}'
        
        
        
        default_html_content = (
            'Try {{try_number}} out of {{max_tries + 1}}<br>'
            'Exception:<br>{{exception_html}}<br>'
            'Log: <a href="{{ti.log_url}}">Link</a><br>'
            'Host: {{ti.hostname}}<br>'
            'Log file: {{ti.log_filepath}}<br>'
            'Mark success: <a href="{{ti.mark_success_url}}">Link</a><br>'
        )
        default_html_content_err = (
            'Try {{try_number}} out of {{max_tries + 1}}<br>'
            'Exception:<br>Failed attempt to attach error logs<br>'
            'Log: <a href="{{ti.log_url}}">Link</a><br>'
            'Host: {{ti.hostname}}<br>'
            'Log file: {{ti.log_filepath}}<br>'
            'Mark success: <a href="{{ti.mark_success_url}}">Link</a><br>'
        )