### 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():
            return None
        task = self.run_as_task(f)
        assert task.execute_callable() is None
    def test_return_false(self):
        def f():
            return False
        task = self.run_as_task(f)
        assert task.execute_callable() is False
    def test_lambda(self):
        with pytest.raises(AirflowException):
            PythonVirtualenvOperator(python_callable=lambda x: 4, task_id=self.task_id)
    def test_nonimported_as_arg(self):
        def f(_):
            return None
        self.run_as_task(f, op_args=[datetime.utcnow()])
    def test_context(self):
        def f(templates_dict):
            return templates_dict["ds"]
        task = self.run_as_task(f, templates_dict={"ds": "{{ ds }}"})
        assert task.templates_dict == {"ds": self.ds_templated}
    def test_deepcopy(self):
        """Test that PythonVirtualenvOperator are deep-copyable."""
        def f():
            return 1
        task = PythonVirtualenvOperator(python_callable=f, task_id="task")
        copy.deepcopy(task)
    def test_virtualenv_serializable_context_fields(self, create_task_instance):