### 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)

import pickle
import shutil
import subprocess
import sys
import types
import warnings
from abc import ABCMeta, abstractmethod
from collections.abc import Container
from pathlib import Path
from tempfile import TemporaryDirectory
from textwrap import dedent
from typing import TYPE_CHECKING, Any, Callable, Collection, Iterable, Mapping, Sequence, cast
import dill
from airflow.exceptions import (
    AirflowConfigException,
    AirflowException,
    AirflowSkipException,
    DeserializingResultError,
    RemovedInAirflow3Warning,
)
from airflow.models.baseoperator import BaseOperator
from airflow.models.skipmixin import SkipMixin
from airflow.models.taskinstance import _CURRENT_CONTEXT
from airflow.utils.context import Context, context_copy_partial, context_merge
from airflow.utils.operator_helpers import KeywordParameters
from airflow.utils.process_utils import execute_in_subprocess
from airflow.utils.python_virtualenv import prepare_virtualenv, write_python_script
if TYPE_CHECKING:
    from pendulum.datetime import DateTime
def task(python_callable: Callable | None = None, multiple_outputs: bool | None = None, **kwargs):