### 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)

self.dag_id = f"dag_{slugify(request.cls.__name__)}"
        self.task_id = f"task_{slugify(request.node.name, max_length=40)}"
        self.run_id = f"run_{slugify(request.node.name, max_length=40)}"
        self.ds_templated = self.default_date.date().isoformat()
        self.ti_maker = create_task_instance_of_operator
        self.dag_maker = dag_maker
        self.dag = self.dag_maker(self.dag_id, template_searchpath=TEMPLATE_SEARCHPATH).dag
        clear_db_runs()
        yield
        clear_db_runs()
    @staticmethod
    def assert_expected_task_states(dag_run: DagRun, expected_states: dict):
        """Helper function that asserts `TaskInstances` of a given `task_id` are in a given state."""
        asserts = []
        for ti in dag_run.get_task_instances():
            try:
                expected = expected_states[ti.task_id]
            except KeyError:
                asserts.append(f"Unexpected task id {ti.task_id!r} found, expected {expected_states.keys()}")
                continue
            if ti.state != expected:
                asserts.append(f"Task {ti.task_id!r} has state {ti.state!r} instead of expected {expected!r}")
        if asserts:
            pytest.fail("\n".join(asserts))
    @staticmethod
    def default_kwargs(**kwargs):