### 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)

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):
        python_path = Path(self.python)
        if not python_path.exists():
            raise ValueError(f"Python Path '{python_path}' must exists")
        if not python_path.is_file():
            raise ValueError(f"Python Path '{python_path}' must be a file")
        if not python_path.is_absolute():
            raise ValueError(f"Python Path '{python_path}' must be an absolute path.")
        python_version_as_list_of_strings = self._get_python_version_from_environment()
        if (
            python_version_as_list_of_strings
            and str(python_version_as_list_of_strings[0]) != str(sys.version_info.major)
            and (self.op_args or self.op_kwargs)
        ):
            raise AirflowException(
                "Passing op_args or op_kwargs is not supported across different Python "
                "major versions for ExternalPythonOperator. Please use string_args."
                f"Sys version: {sys.version_info}. Venv version: {python_version_as_list_of_strings}"
            )
        with TemporaryDirectory(prefix="tmd") as tmp_dir:
            tmp_path = Path(tmp_dir)
            return self._execute_python_callable_in_subprocess(python_path, tmp_path)
    def _get_python_version_from_environment(self) -> list[str]: