Fixed `create-release-branch` failing with non-core tags
Fixed `create-release-branch` failing with non-core tags

- the create-release-branch workflow works by getting the most recent
  tag, bumping it and using that to create the new tag
- now we've moved Portal into the monorepo, we've got two different
  types of flags, but the Portal ones aren't valid semver so the
  workflow fails if the most recent tag is one for Portal
- this fixes that by ensuring we only fetch tags matching the pattern we
  use for tagging Ghost

TryGhost/Ghost,TryGhost/Ghost,TryGhost/Ghost