### 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)

"""Create TaskInstance and run it."""
        ti = self.create_ti(fn, **kwargs)
        ti.run()
        if return_ti:
            return ti
        return ti.task
    def render_templates(self, fn, **kwargs):
        """Create TaskInstance and render templates without actual run."""
        return self.create_ti(fn, **kwargs).render_templates()
class TestPythonOperator(BasePythonTest):
    opcls = PythonOperator
    @pytest.fixture(autouse=True)
    def setup_tests(self):
        self.run = False
    def do_run(self):
        self.run = True
    def is_run(self):
        return self.run
    def test_python_operator_run(self):
        """Tests that the python callable is invoked on task run."""
        ti = self.create_ti(self.do_run)
        assert not self.is_run()
        ti.run()
        assert self.is_run()
    @pytest.mark.parametrize("not_callable", [{}, None])
    def test_python_operator_python_callable_is_callable(self, not_callable):