### 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)

virtualenv_string_args: list[str] = []
class BaseTestPythonVirtualenvOperator(BasePythonTest):
    def test_template_fields(self):
        assert set(PythonOperator.template_fields).issubset(PythonVirtualenvOperator.template_fields)
    def test_fail(self):
        def f():
            raise Exception
        with pytest.raises(CalledProcessError):
            self.run_as_task(f)
    def test_fail_with_message(self):
        def f():
            raise Exception("Custom error message")
        with pytest.raises(AirflowException) as e:
            self.run_as_task(f)
            assert "Custom error message" in str(e)
    def test_string_args(self):
        def f():
            global virtualenv_string_args
            print(virtualenv_string_args)
            if virtualenv_string_args[0] != virtualenv_string_args[2]:
                raise Exception
        self.run_as_task(f, string_args=[1, 2, 1])
    def test_with_args(self):
        def f(a, b, c=False, d=False):
            if a == 0 and b == 1 and c and not d:
                return True
            else:
                raise Exception
        self.run_as_task(f, op_args=[0, 1], op_kwargs={"c": True})
    def test_return_none(self):