### 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)

if self.ignore_downstream_trigger_rules is True:
                tasks = context["task"].get_flat_relatives(upstream=False)
            else:
                tasks = context["task"].get_direct_relatives(upstream=False)
            for t in tasks:
                if not t.is_teardown:
                    yield t
        to_skip = get_tasks_to_skip()
        
        if self.log.getEffectiveLevel() <= logging.DEBUG:
            self.log.debug("Downstream task IDs %s", to_skip := list(get_tasks_to_skip()))
        self.log.info("Skipping downstream tasks")
        self.skip(
            dag_run=dag_run,
            execution_date=cast("DateTime", dag_run.execution_date),
            tasks=to_skip,
            map_index=context["ti"].map_index,
        )
        self.log.info("Done.")
        
        return condition
class _BasePythonVirtualenvOperator(PythonOperator, metaclass=ABCMeta):
    BASE_SERIALIZABLE_CONTEXT_KEYS = {
        "ds",
        "ds_nodash",
        "expanded_ti_count",
        "inlets",