fix: specify local protobuf generator ci template to publish on tags

Part of #3 (closed)

This matches what we are currently doing in https://gitlab.developers.cam.ac.uk/uis/devops/iam/activate-account/api/-/blob/main/.gitlab/openapi-generator-local.gitlab-ci.yml?ref_type=heads&plain=1, which hopefully fixes the fact that protobuf:publish:python:gitlab won't run.