### 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(templates_dict):
            return templates_dict["ds"]
        with pytest.raises(AirflowException, match="Invalid tasks found:"):
            self.run_as_task(f, templates_dict={"ds": "{{ ds }}"})
    def test_with_dag_run(self):
        with self.dag:
            def f():
                return "branch_1"
            branch_op = self.opcls(task_id=self.task_id, python_callable=f, **self.default_kwargs())
            branch_op >> [self.branch_1, self.branch_2]
        dr = self.create_dag_run()
        branch_op.run(start_date=self.default_date, end_date=self.default_date)
        self.assert_expected_task_states(
            dr, {self.task_id: State.SUCCESS, "branch_1": State.NONE, "branch_2": State.SKIPPED}
        )
    def test_with_skip_in_branch_downstream_dependencies(self):
        with self.dag:
            def f():
                return "branch_1"
            branch_op = self.opcls(task_id=self.task_id, python_callable=f, **self.default_kwargs())
            branch_op >> self.branch_1 >> self.branch_2
            branch_op >> self.branch_2
        dr = self.create_dag_run()
        branch_op.run(start_date=self.default_date, end_date=self.default_date)
        self.assert_expected_task_states(
            dr, {self.task_id: State.SUCCESS, "branch_1": State.NONE, "branch_2": State.NONE}
        )
    def test_with_skip_in_branch_downstream_dependencies2(self):
        with self.dag:
            def f():