Avoid race condition in CI during Github releases

[jluethi]

The newly added feature of triggering the CI during Github releases (see #20) sometimes leads to the following race condition:

If the Github release is made before the CI in main has finished running, the CI in main will then deploy the whls to the Github release & the PyPI. However, the release also triggers the CI to run again (necessary because the CI in main my have finished & there was no tag when that ran).
In that case, the publish CI run can run into errors like:
Publish to PyPI fails:

Uploading distributions to https://upload.pypi.org/legacy/
Uploading fractal_lif_converters-0.3.4-py3-none-any.whl
Uploading fractal_lif_converters-0.3.4.tar.gz
WARNING  Error during upload. Retry with the --verbose option for more details. 
ERROR    HTTPError: 400 Bad Request from https://upload.pypi.org/legacy/        
         The digest supplied does not match a digest calculated from the        
         uploaded file.  

or adding to release fails:

Found release v0.1.3 (with id=190475854)
⬆️ Uploading fractal_helper_tasks-0.1.3.tar.gz...
⬆️ Uploading fractal_helper_tasks-0.1.3-py3-none-any.whl...
Error: Validation Failed: {"resource":"ReleaseAsset","code":"already_exists","field":"name"} - https://docs.github.com/rest