### 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)

try:
            result = subprocess.check_output([self.python, "--version"], text=True)
            return result.strip().split(" ")[-1].split(".")
        except Exception as e:
            raise ValueError(f"Error while executing {self.python}: {e}")
    def _iter_serializable_context_keys(self):
        yield from self.BASE_SERIALIZABLE_CONTEXT_KEYS
        if self._get_airflow_version_from_target_env():
            yield from self.AIRFLOW_SERIALIZABLE_CONTEXT_KEYS
            yield from self.PENDULUM_SERIALIZABLE_CONTEXT_KEYS
        elif self._is_pendulum_installed_in_target_env():
            yield from self.PENDULUM_SERIALIZABLE_CONTEXT_KEYS
    def _is_pendulum_installed_in_target_env(self) -> bool:
        try:
            subprocess.check_call([self.python, "-c", "import pendulum"])
            return True
        except Exception as e:
            if self.expect_pendulum:
                self.log.warning("When checking for Pendulum installed in venv got %s", e)
                self.log.warning(
                    "Pendulum is not properly installed in the virtualenv "
                    "Pendulum context keys will not be available. "
                    "Please Install Pendulum or Airflow in your venv to access them."
                )
            return False
    def _get_airflow_version_from_target_env(self) -> str | None: