**Apache Airflow version**: 2.1.0

**What happened**:

Using a Label infront of a TaskGroup breaks task dependencies, as tasks before the TaskGroup will now point to the last task in the TaskGroup.

**What you expected to happen**:

Task dependencies with or without Labels should be the same (and preferably the Label should be visible outside of the TaskGroup).
Behavior without a Label is as follows:

```python
with TaskGroup("tg1", tooltip="Tasks related to task group") as tg1:
        DummyOperator(task_id="b") >> DummyOperator(task_id="c")

DummyOperator(task_id="a") >> tg1
```
![without Taskgroup](https://user-images.githubusercontent.com/35238779/128502690-58b31577-91d7-4b5c-b348-4779c9286958.png)

**How to reproduce it**:

```python
with TaskGroup("tg1", tooltip="Tasks related to task group") as tg1:
        DummyOperator(task_id="b") >> DummyOperator(task_id="c")

    DummyOperator(task_id="a") >> Label("testlabel") >> tg1
```
![with TaskGroup](https://user-images.githubusercontent.com/35238779/128502758-5252b7e7-7c30-424a-ac3a-3f9a99b0a805.png)


TaskGroupContext.push_context_managed_task_group(self)
        return self
    def __exit__(self, _type, _value, _tb):
        TaskGroupContext.pop_context_managed_task_group()
    def has_task(self, task: BaseOperator) -> bool:
        """Returns True if this TaskGroup or its children TaskGroups contains the given task."""
        if task.task_id in self.children:
            return True
        return any(child.has_task(task) for child in self.children.values() if isinstance(child, TaskGroup))
    @property
    def roots(self) -> list[BaseOperator]:
        """Required by TaskMixin"""
        return list(self.get_roots())
    @property
    def leaves(self) -> list[BaseOperator]: