### 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)

"a_templated_string": "dag {{dag.dag_id}} ran on {{ds}}.",
            },
        )
        rendered_op_kwargs = task.op_kwargs
        assert rendered_op_kwargs["an_int"] == 4
        assert rendered_op_kwargs["a_date"] == date(2019, 1, 1)
        assert rendered_op_kwargs["a_templated_string"] == f"dag {self.dag_id} ran on {self.ds_templated}."
    def test_python_operator_shallow_copy_attr(self):
        def not_callable(x):
            assert False, "Should not be triggered"
        original_task = PythonOperator(
            python_callable=not_callable,
            op_kwargs={"certain_attrs": ""},
            task_id=self.task_id,
        )
        new_task = copy.deepcopy(original_task)
        
        assert id(original_task.op_kwargs["certain_attrs"]) == id(new_task.op_kwargs["certain_attrs"])
        
        assert id(original_task.python_callable) == id(new_task.python_callable)
    def test_conflicting_kwargs(self):
        
        def func(dag):
            
            raise RuntimeError(f"Should not be triggered, dag: {dag}")
        ti = self.create_ti(func, op_args=[1])
        error_message = re.escape("The key 'dag' in args is a part of kwargs and therefore reserved.")
        with pytest.raises(ValueError, match=error_message):
            ti.run()
    def test_provide_context_does_not_fail(self):