### 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)

[ti for ti in tis if ti.task_id == "op1"], session=session, dag=short_circuit.dag
            )
        self.op1.run(start_date=self.default_date, end_date=self.default_date)
        self.assert_expected_task_states(dr, expected_states)
    def test_xcom_push(self):
        with self.dag:
            short_op_push_xcom = ShortCircuitOperator(
                task_id="push_xcom_from_shortcircuit", python_callable=lambda: "signature"
            )
            short_op_no_push_xcom = ShortCircuitOperator(
                task_id="do_not_push_xcom_from_shortcircuit", python_callable=lambda: False
            )
        dr = self.create_dag_run()
        short_op_push_xcom.run(start_date=self.default_date, end_date=self.default_date)
        short_op_no_push_xcom.run(start_date=self.default_date, end_date=self.default_date)
        tis = dr.get_task_instances()
        assert tis[0].xcom_pull(task_ids=short_op_push_xcom.task_id, key="return_value") == "signature"
        assert tis[0].xcom_pull(task_ids=short_op_no_push_xcom.task_id, key="return_value") is False
    def test_xcom_push_skipped_tasks(self):
        with self.dag:
            short_op_push_xcom = ShortCircuitOperator(
                task_id="push_xcom_from_shortcircuit", python_callable=lambda: False
            )
            empty_task = EmptyOperator(task_id="empty_task")
            short_op_push_xcom >> empty_task
        dr = self.create_dag_run()
        short_op_push_xcom.run(start_date=self.default_date, end_date=self.default_date)
        tis = dr.get_task_instances()
        assert tis[0].xcom_pull(task_ids=short_op_push_xcom.task_id, key="skipmixin_key") == {
            "skipped": ["empty_task"]