### 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)

def f(
            
            ds_nodash,
            inlets,
            next_ds,
            next_ds_nodash,
            outlets,
            prev_ds,
            prev_ds_nodash,
            run_id,
            task_instance_key_str,
            test_mode,
            tomorrow_ds,
            tomorrow_ds_nodash,
            ts,
            ts_nodash,
            ts_nodash_with_tz,
            yesterday_ds,
            yesterday_ds_nodash,
            
            execution_date,
            next_execution_date,
            prev_execution_date,
            prev_execution_date_success,
            prev_start_date_success,
            
            **context,