### 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)

"""Ensure all template context fields are listed in the operator.
        This exists mainly so when a field is added to the context, we remember to
        also add it to PythonVirtualenvOperator.
        """
        
        
        
        intentionally_excluded_context_keys = [
            "task_instance",
            "ti",
            "var",  
            "conn",  
        ]
        ti = create_task_instance(dag_id=self.dag_id, task_id=self.task_id, schedule=None)
        context = ti.get_template_context()
        declared_keys = {
            *PythonVirtualenvOperator.BASE_SERIALIZABLE_CONTEXT_KEYS,
            *PythonVirtualenvOperator.PENDULUM_SERIALIZABLE_CONTEXT_KEYS,
            *PythonVirtualenvOperator.AIRFLOW_SERIALIZABLE_CONTEXT_KEYS,
            *intentionally_excluded_context_keys,
        }
        assert set(context) == declared_keys
    @pytest.mark.parametrize(
        "extra_kwargs, actual_exit_code, expected_state",
        [
            (None, 99, TaskInstanceState.FAILED),
            ({"skip_on_exit_code": 100}, 100, TaskInstanceState.SKIPPED),
            ({"skip_on_exit_code": [100]}, 100, TaskInstanceState.SKIPPED),