Fix notifications in Assets GitHub Actions

The slack notifications are broken in the GitHub actions. They cause the actions to appear to have failed even though they have really succeeded, for example, releases appear to have failed when they have not, as shown in the screengrab:

<img width="1170" alt="image" src="https://github.com/datacite/assets/assets/4681304/f7a79d62-a1b9-47d4-80b8-34afcc6fafc3">
