### 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)

return "branch_1"
            branch_op = self.opcls(task_id=self.task_id, python_callable=f, **self.default_kwargs())
            branches = [self.branch_1, self.branch_2]
            branch_op >> branches
        dr = self.create_dag_run()
        branch_op.run(start_date=self.default_date, end_date=self.default_date)
        for task in branches:
            task.run(start_date=self.default_date, end_date=self.default_date)
        expected_states = {
            self.task_id: State.SUCCESS,
            "branch_1": State.SUCCESS,
            "branch_2": State.SKIPPED,
        }
        self.assert_expected_task_states(dr, expected_states)
        
        tis = dr.get_task_instances()
        children_tis = [ti for ti in tis if ti.task_id in branch_op.get_direct_relative_ids()]
        with create_session() as session:
            clear_task_instances(children_tis, session=session, dag=branch_op.dag)
        
        for task in branches:
            task.run(start_date=self.default_date, end_date=self.default_date)
        
        self.assert_expected_task_states(dr, expected_states)
    def test_raise_exception_on_no_accepted_type_return(self):