### 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)

caplog.set_level(logging.INFO, logger=LOGGER_NAME)
        def func():
            return "test_return_value"
        if show_return_value_in_logs is NOTSET:
            self.run_as_task(func)
        else:
            self.run_as_task(func, show_return_value_in_logs=show_return_value_in_logs)
        if should_shown:
            assert "Done. Returned value was: test_return_value" in caplog.messages
            assert "Done. Returned value not shown" not in caplog.messages
        else:
            assert "Done. Returned value was: test_return_value" not in caplog.messages
            assert "Done. Returned value not shown" in caplog.messages
    def test_python_operator_templates_exts(self):
        def func():
            return "test_return_value"
        python_operator = PythonOperator(
            task_id="python_operator",
            python_callable=func,
            dag=self.dag,
            show_return_value_in_logs=False,
            templates_exts=["test_ext"],
        )
        assert python_operator.template_ext == ["test_ext"]
class TestBranchOperator(BasePythonTest):
    opcls = BranchPythonOperator
    @pytest.fixture(autouse=True)
    def setup_tests(self):