**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)


self,
        task_or_task_list: DependencyMixin | Sequence[DependencyMixin],
        upstream: bool = False,
        edge_modifier: EdgeModifier | None = None,
    ) -> None:
        """
        Call set_upstream/set_downstream for all root/leaf tasks within this TaskGroup.
        Update upstream_group_ids/downstream_group_ids/upstream_task_ids/downstream_task_ids.
        """
        if not isinstance(task_or_task_list, Sequence):
            task_or_task_list = [task_or_task_list]
        for task_like in task_or_task_list:
            self.update_relative(task_like, upstream)
        if upstream:
            for task in self.get_roots():
                task.set_upstream(task_or_task_list)
        else:
            for task in self.get_leaves():
                task.set_downstream(task_or_task_list)
    def __enter__(self) -> TaskGroup: