Follow-up to #608

Adds a fetch of the release branches so that we can figure out the release branch name that the tag corresponds to. This is required because the default checkout behaviour of the actions/checkout step is to fetch only the single commit without any branch names.

An alternative to this is to specify fetch-depth: 0 on the actions/checkout step which will result in a full fetch of all commits, branches, and tags. However, that is slightly slower due to it fetching everything instead of the more minimal set of commit data. And would apply to every build the way it's setup now and not only for the publish.