### 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)

opcls = ExternalPythonOperator
    @staticmethod
    def default_kwargs(*, python_version=sys.version_info[0], **kwargs):
        kwargs["python"] = sys.executable
        return kwargs
    @mock.patch("pickle.loads")
    def test_except_value_error(self, loads_mock):
        def f():
            return 1
        task = PythonVirtualenvOperator(
            python_callable=f,
            task_id="task",
            dag=self.dag,
        )
        loads_mock.side_effect = DeserializingResultError
        with pytest.raises(DeserializingResultError):
            task._read_result(path=mock.Mock())
class TestExternalBranchPythonOperator(BaseTestPythonVirtualenvOperator):