### 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)

self,
        *,
        python_callable: Callable,
        op_args: Collection[Any] | None = None,
        op_kwargs: Mapping[str, Any] | None = None,
        templates_dict: dict[str, Any] | None = None,
        templates_exts: Sequence[str] | None = None,
        show_return_value_in_logs: bool = True,
        **kwargs,
    ) -> None:
        if kwargs.get("provide_context"):
            warnings.warn(
                "provide_context is deprecated as of 2.0 and is no longer required",
                RemovedInAirflow3Warning,
                stacklevel=2,
            )
            kwargs.pop("provide_context", None)
        super().__init__(**kwargs)
        if not callable(python_callable):
            raise AirflowException("`python_callable` param must be callable")
        self.python_callable = python_callable
        self.op_args = op_args or ()
        self.op_kwargs = op_kwargs or {}
        self.templates_dict = templates_dict
        if templates_exts:
            self.template_ext = templates_exts
        self.show_return_value_in_logs = show_return_value_in_logs
    def execute(self, context: Context) -> Any: