### 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)

({"skip_on_exit_code": (100, 101)}, 100, TaskInstanceState.SKIPPED),
            ({"skip_on_exit_code": 100}, 101, TaskInstanceState.FAILED),
            ({"skip_on_exit_code": [100, 102]}, 101, TaskInstanceState.FAILED),
            ({"skip_on_exit_code": None}, 0, TaskInstanceState.SUCCESS),
        ],
    )
    def test_on_skip_exit_code(self, extra_kwargs, actual_exit_code, expected_state):
        def f(exit_code):
            if exit_code != 0:
                raise SystemExit(exit_code)
        if expected_state == TaskInstanceState.FAILED:
            with pytest.raises(CalledProcessError):
                self.run_as_task(
                    f, op_kwargs={"exit_code": actual_exit_code}, **(extra_kwargs if extra_kwargs else {})
                )
        else:
            ti = self.run_as_task(
                f,
                return_ti=True,
                op_kwargs={"exit_code": actual_exit_code},
                **(extra_kwargs if extra_kwargs else {}),
            )
            assert ti.state == expected_state
class TestPythonVirtualenvOperator(BaseTestPythonVirtualenvOperator):
    opcls = PythonVirtualenvOperator
    @staticmethod
    def default_kwargs(*, python_version=sys.version_info[0], **kwargs):
        kwargs["python_version"] = python_version
        return kwargs
    def test_add_dill(self):