### 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)

python_callable_source=self.get_python_source(),
            ),
            filename=os.fspath(script_path),
            render_template_as_native_obj=self.dag.render_template_as_native_obj,
        )
        try:
            execute_in_subprocess(
                cmd=[
                    os.fspath(python_path),
                    os.fspath(script_path),
                    os.fspath(input_path),
                    os.fspath(output_path),
                    os.fspath(string_args_path),
                    os.fspath(termination_log_path),
                ]
            )
        except subprocess.CalledProcessError as e:
            if e.returncode in self.skip_on_exit_code:
                raise AirflowSkipException(f"Process exited with code {e.returncode}. Skipping.")
            elif termination_log_path.exists() and termination_log_path.stat().st_size > 0:
                error_msg = f"Process returned non-zero exit status {e.returncode}.\n"
                with open(termination_log_path) as file:
                    error_msg += file.read()
                raise AirflowException(error_msg) from None
            else:
                raise
        return self._read_result(output_path)
    def determine_kwargs(self, context: Mapping[str, Any]) -> Mapping[str, Any]:
        return KeywordParameters.determine(self.python_callable, self.op_args, context).serializing()
class PythonVirtualenvOperator(_BasePythonVirtualenvOperator):