push to GAR action writes files to the repository's working directory

Back in April [it was reported](https://raintank-corp.slack.com/archives/C0671T2J5CH/p1744106839648409) that one of the shared workflows was writing stuff to the working directory while running. [This PR](https://github.com/grafana/shared-workflows/pull/894) was mentioned in that conversation.

It was then reported that push to GAR was creating a `shared-workflows` directory.

Now it's writing a different directory:

```
#16 [linux/amd64->arm64 buildtools 4/5] RUN git status && git clean -dxn
#16 0.081 HEAD detached at pull/714/merge
#16 0.081 Untracked files:
#16 0.081 (use "git add ..." to include in what will be committed)
#16 0.081 _shared-workflows-push-to-gar/
#16 0.081
#16 0.081 nothing added to commit but untracked files present (use "git add" to track)
#16 DONE 0.1s
```

It's creating a `_shared-workflows-push-to-gar/` directory.