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.

.gitlab/protobuf-generator-local.gitlab-ci.yml

+# *REPOSITORY LOCAL* specialisation of the protobuf generator templates.
+
+# Set rules for when we publish protobuf client packages.
+.protobuf:publish:base:
+  rules:
+    # We publish packages for all commits tagged with what looks like a version number.
+    - if: $CI_COMMIT_TAG =~ /[0-9]+\.[0-9]+\.+[0-9]+.*/