### 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)

branch = super().execute(context)
        self.log.info("Branch callable return %s", branch)
        self.skip_all_except(context["ti"], branch)
        return branch
def get_current_context() -> Context:
    """
    Retrieve the execution context dictionary without altering user method's signature.
    This is the simplest method of retrieving the execution context dictionary.
    **Old style:**
    .. code:: python
        def my_task(**context):
            ti = context["ti"]
    **New style:**
    .. code:: python
        from airflow.operators.python import get_current_context
        def my_task():
            context = get_current_context()
            ti = context["ti"]
    Current context will only have value if this method was called after an operator
    was starting to execute.
    """
    if not _CURRENT_CONTEXT:
        raise AirflowException(
            "Current context was requested but no context was found! "
            "Are you running within an airflow task?"
        )
    return _CURRENT_CONTEXT[-1]