### 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)

example_context = {"Hello": "World"}
        with set_current_context(example_context):
            pass
        with pytest.raises(AirflowException):
            get_current_context()
    def test_nested_context(self):
        """
        Nested execution context should be supported in case the user uses multiple context managers.
        Each time the execute method of an operator is called, we set a new 'current' context.
        This test verifies that no matter how many contexts are entered - order is preserved
        """
        max_stack_depth = 15
        ctx_list = []
        for i in range(max_stack_depth):
            
            new_context = {"ContextId": i}
            
            ctx_obj = set_current_context(new_context)
            ctx_obj.__enter__()
            ctx_list.append(ctx_obj)
        for i in reversed(range(max_stack_depth)):
            
            ctx = get_current_context()
            assert ctx["ContextId"] == i
            
            ctx_list[i].__exit__(None, None, None)
class MyContextAssertOperator(BaseOperator):