fix: ensure the reuse job is always run

The .pre stage is not guaranteed to always execute. In particular, if there are no jobs defined/enabled outside the .pre and .post stages then no jobs will be executed at all, regardless of whether there are jobs enabled in .pre or .post. E.g.: a project which does not define custom jobs and only references the ci-reuse.yml template in the .gitlab-ci.yml file would trigger this scenario.

Since ci-reuse.yml is an explicit opt-in, let's ensure that if a project does opt-in it will always run.

See-Also: https://docs.gitlab.com/12.10/ee/ci/yaml/#pre-and-post