### 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)

"""Tests that PythonOperator will only instantiate if the python_callable argument is callable."""
        with pytest.raises(AirflowException, match="`python_callable` param must be callable"):
            PythonOperator(python_callable=not_callable, task_id="python_operator")
    def test_python_callable_arguments_are_templatized(self):
        """Test PythonOperator op_args are templatized"""
        
        
        Named = namedtuple("Named", ["var1", "var2"])
        named_tuple = Named("{{ ds }}", "unchanged")
        task = self.render_templates(
            lambda: 0,
            op_args=[4, date(2019, 1, 1), "dag {{dag.dag_id}} ran on {{ds}}.", named_tuple],
        )
        rendered_op_args = task.op_args
        assert len(rendered_op_args) == 4
        assert rendered_op_args[0] == 4
        assert rendered_op_args[1] == date(2019, 1, 1)
        assert rendered_op_args[2] == f"dag {self.dag_id} ran on {self.ds_templated}."
        assert rendered_op_args[3] == Named(self.ds_templated, "unchanged")
    def test_python_callable_keyword_arguments_are_templatized(self):
        """Test PythonOperator op_kwargs are templatized"""
        task = self.render_templates(
            lambda: 0,
            op_kwargs={
                "an_int": 4,
                "a_date": date(2019, 1, 1),