### 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)

"""
        Tests that BranchPythonOperator handles empty branches properly.
        """
        with self.dag:
            def f():
                return choice
            branch = self.opcls(task_id=self.task_id, python_callable=f, **self.default_kwargs())
            task1 = EmptyOperator(task_id="task1")
            join = EmptyOperator(task_id="join", trigger_rule="none_failed_min_one_success")
            branch >> [task1, join]
            task1 >> join
        dr = self.create_dag_run()
        task_ids = [self.task_id, "task1", "join"]
        tis = {ti.task_id: ti for ti in dr.task_instances}
        for task_id in task_ids:  
            task_instance = tis[task_id]
            task_instance.refresh_from_task(self.dag.get_task(task_id))
            task_instance.run()
        def get_state(ti):
            ti.refresh_from_db()
            return ti.state
        assert [get_state(tis[task_id]) for task_id in task_ids] == expected_states
class TestShortCircuitOperator(BasePythonTest):
    opcls = ShortCircuitOperator
    @pytest.fixture(autouse=True)
    def setup_tests(self):