### 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)

):
            raise AirflowException(
                "Passing op_args or op_kwargs is not supported across different Python "
                "major versions for PythonVirtualenvOperator. Please use string_args."
                f"Sys version: {sys.version_info}. Venv version: {python_version}"
            )
        if not shutil.which("virtualenv"):
            raise AirflowException("PythonVirtualenvOperator requires virtualenv, please install it.")
        if not requirements:
            self.requirements: list[str] | str = []
        elif isinstance(requirements, str):
            self.requirements = requirements
        else:
            self.requirements = list(requirements)
        self.python_version = python_version
        self.system_site_packages = system_site_packages
        self.pip_install_options = pip_install_options
        super().__init__(
            python_callable=python_callable,
            use_dill=use_dill,
            op_args=op_args,
            op_kwargs=op_kwargs,
            string_args=string_args,
            templates_dict=templates_dict,
            templates_exts=templates_exts,
            expect_airflow=expect_airflow,
            skip_on_exit_code=skip_on_exit_code,
            **kwargs,
        )
    def execute_callable(self):