### Apache Airflow version

2.3.0

### What happened

When using a `PythonVirtualenvOperator` it is checked if `virtualenv` is installed by 
`if not shutil.which("virtualenv"):`
https://github.com/apache/airflow/blob/a1679be85aa49c0d6a7ba2c31acb519a5bcdf594/airflow/operators/python.py#L398
Actually, this expression checks if `virtualenv` is on PATH. If Airflow is installed in a virtual environment itself and `virtualenv` is not installed in the environment the check might pass but `virtualenv` cannot be used as it is not present in the environment.

### What you think should happen instead

It should be checked if `virtualenv` is actually available in the environment.

```python
if importlib.util.find_spec("virtualenv") is None:
    raise AirflowException('PythonVirtualenvOperator requires virtualenv, please install it.')
```
https://stackoverflow.com/a/14050282

### How to reproduce

_No response_

### Operating System

Ubuntu 20.04

### Versions of Apache Airflow Providers

_No response_

### Deployment

Virtualenv installation

### Deployment details

_No response_

### Anything else

_No response_

### Are you willing to submit PR?

- [X] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

}
    def test_mapped_xcom_push_skipped_tasks(self, session):
        with self.dag:
            @task_group
            def group(x):
                short_op_push_xcom = ShortCircuitOperator(
                    task_id="push_xcom_from_shortcircuit",
                    python_callable=lambda arg: arg % 2 == 0,
                    op_kwargs={"arg": x},
                )
                empty_task = EmptyOperator(task_id="empty_task")
                short_op_push_xcom >> empty_task
            group.expand(x=[0, 1])
        dr = self.create_dag_run()
        decision = dr.task_instance_scheduling_decisions(session=session)
        for ti in decision.schedulable_tis:
            ti.run()
        
        tis = dr.get_task_instances()
        assert (
            tis[0].xcom_pull(task_ids="group.push_xcom_from_shortcircuit", key="return_value", map_indexes=0)
            is True
        )
        assert (
            tis[0].xcom_pull(task_ids="group.push_xcom_from_shortcircuit", key="skipmixin_key", map_indexes=0)
            is None
        )
        assert tis[0].xcom_pull(
            task_ids="group.push_xcom_from_shortcircuit", key="skipmixin_key", map_indexes=1
        ) == {"skipped": ["group.empty_task"]}