As reported by @Deexie, during the process of opening backport PRs in https://github.com/scylladb/scylladb/pull/21616, No invite emails were sent, causing a lack of permissions for the backport PR branch
The check if `has_in_collaborators(pr.user.login)` was pointing to `scylladb/scylladb` instead of `scylladbbot/scylladb`, fixing it
I also moved the collaborator check to an early stage, before trying to open a backport PR
Closesscylladb/scylladb#21645