### 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)

ExternalBranchPythonOperator,
    ExternalPythonOperator,
    PythonOperator,
    PythonVirtualenvOperator,
    ShortCircuitOperator,
    get_current_context,
)
from airflow.utils import timezone
from airflow.utils.context import AirflowContextDeprecationWarning, Context
from airflow.utils.python_virtualenv import prepare_virtualenv
from airflow.utils.session import create_session
from airflow.utils.state import DagRunState, State, TaskInstanceState
from airflow.utils.trigger_rule import TriggerRule
from airflow.utils.types import NOTSET, DagRunType
from tests.test_utils import AIRFLOW_MAIN_FOLDER
from tests.test_utils.db import clear_db_runs
DEFAULT_DATE = timezone.datetime(2016, 1, 1)
TEMPLATE_SEARCHPATH = os.path.join(AIRFLOW_MAIN_FOLDER, "tests", "config_templates")
LOGGER_NAME = "airflow.task.operators"
class BasePythonTest:
    """Base test class for TestPythonOperator and TestPythonSensor classes"""
    opcls: type[BaseOperator]
    dag_id: str
    task_id: str
    run_id: str
    dag: DAG
    ds_templated: str
    default_date: datetime = DEFAULT_DATE
    @pytest.fixture(autouse=True)
    def base_tests_setup(self, request, create_task_instance_of_operator, dag_maker):