### 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)

"""Ensures that provide_context doesn't break dags in 2.0."""
        def func(custom, dag):
            assert 1 == custom, "custom should be 1"
            assert dag is not None, "dag should be set"
        with pytest.warns(RemovedInAirflow3Warning):
            self.run_as_task(func, op_kwargs={"custom": 1}, provide_context=True)
    def test_context_with_conflicting_op_args(self):
        def func(custom, dag):
            assert 1 == custom, "custom should be 1"
            assert dag is not None, "dag should be set"
        self.run_as_task(func, op_kwargs={"custom": 1})
    def test_context_with_kwargs(self):
        def func(**context):
            
            assert len(context) > 0, "Context has not been injected"
        self.run_as_task(func, op_kwargs={"custom": 1})
    @pytest.mark.parametrize(
        "show_return_value_in_logs, should_shown",
        [
            pytest.param(NOTSET, True, id="default"),
            pytest.param(True, True, id="show"),
            pytest.param(False, False, id="hide"),
        ],
    )
    def test_return_value_log(self, show_return_value_in_logs, should_shown, caplog):