### 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)

opcls = ExternalBranchPythonOperator
    @pytest.fixture(autouse=True)
    def setup_tests(self):
        self.branch_1 = EmptyOperator(task_id="branch_1")
        self.branch_2 = EmptyOperator(task_id="branch_2")
    @staticmethod
    def default_kwargs(*, python_version=sys.version_info[0], **kwargs):
        kwargs["python"] = sys.executable
        return kwargs
    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
        with pytest.raises(AirflowException, match="but got 'bool'"):
            self.run_as_task(f, op_args=[0, 1], op_kwargs={"c": True})
    def test_return_false(self):
        def f():
            return False
        with pytest.raises(AirflowException, match="but got 'bool'"):
            self.run_as_task(f)
    def test_context(self):