### 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)

assert short_circuit.trigger_rule == TriggerRule.ALL_SUCCESS
        assert self.op1.trigger_rule == TriggerRule.ALL_SUCCESS
        assert self.op2.trigger_rule == test_trigger_rule
        self.assert_expected_task_states(dr, expected_task_states)
    def test_clear_skipped_downstream_task(self):
        """
        After a downstream task is skipped by ShortCircuitOperator, clearing the skipped task
        should not cause it to be executed.
        """
        with self.dag:
            short_circuit = ShortCircuitOperator(task_id="short_circuit", python_callable=lambda: False)
            short_circuit >> self.op1 >> self.op2
        dr = self.create_dag_run()
        short_circuit.run(start_date=self.default_date, end_date=self.default_date)
        self.op1.run(start_date=self.default_date, end_date=self.default_date)
        self.op2.run(start_date=self.default_date, end_date=self.default_date)
        assert short_circuit.ignore_downstream_trigger_rules
        assert short_circuit.trigger_rule == TriggerRule.ALL_SUCCESS
        assert self.op1.trigger_rule == TriggerRule.ALL_SUCCESS
        assert self.op2.trigger_rule == TriggerRule.ALL_SUCCESS
        expected_states = {
            "short_circuit": State.SUCCESS,
            "op1": State.SKIPPED,
            "op2": State.SKIPPED,
        }
        self.assert_expected_task_states(dr, expected_states)
        
        tis = dr.get_task_instances()
        with create_session() as session:
            clear_task_instances(