### 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)

def test_context_in_task(self):
        with DAG(dag_id="assert_context_dag", default_args=DEFAULT_ARGS, schedule="@once"):
            op = MyContextAssertOperator(task_id="assert_context")
            op.run(ignore_first_depends_on_past=True, ignore_ti_state=True)
    def test_get_context_in_old_style_context_task(self):
        with DAG(dag_id="edge_case_context_dag", default_args=DEFAULT_ARGS, schedule="@once"):
            op = PythonOperator(python_callable=get_all_the_context, task_id="get_all_the_context")
            op.run(ignore_first_depends_on_past=True, ignore_ti_state=True)
class TestShortCircuitWithTeardown:
    @pytest.mark.parametrize(
        "ignore_downstream_trigger_rules, with_teardown, should_skip, expected",
        [
            (False, True, True, ["op2"]),
            (False, True, False, []),
            (False, False, True, ["op2"]),
            (False, False, False, []),
            (True, True, True, ["op2", "op3"]),
            (True, True, False, []),
            (True, False, True, ["op2", "op3", "op4"]),
            (True, False, False, []),