Incremental build

Changes the incremental build script so it looks at the latest tag from the release branch to build against. We might have to change this in the future, if we actually start doing sub-versions and specific release branches again, but for now, this should be good. This doesn't really work with forks in any case, but I can't come up with a good way to circumvent that. The job remains allow_failure: yes, so even if it breaks, it should not be a problem.