### 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)

def f():
            return 5
        ti = self.create_ti(f)
        with pytest.raises(AirflowException, match="must be either None, a task ID, or an Iterable of IDs"):
            ti.run()
    def test_raise_exception_on_invalid_task_id(self):
        def f():
            return "some_task_id"
        ti = self.create_ti(f)
        with pytest.raises(AirflowException, match="Invalid tasks found: {'some_task_id'}"):
            ti.run()
class TestCurrentContext:
    def test_current_context_no_context_raise(self):
        with pytest.raises(AirflowException):
            get_current_context()
    def test_current_context_roundtrip(self):
        example_context = {"Hello": "World"}
        with set_current_context(example_context):
            assert get_current_context() == example_context
    def test_context_removed_after_exit(self):