.gitlab-ci.yml: Enable security repo to run rails tests

MRs on the gitaly project can currently kick off a rails pipeline that runs the rails test suite with the GITALY_SERVER_VERSION set to the Gitaly change in order to confirm that the change hasn't broken any Rails tests.

However, we don't have this capability on the security repo, which leads to problems such as https://gitlab.com/gitlab-org/security/gitlab/-/pipelines/1130913462, where a security MR on the gitaly repo broke Rails tests.

To fix this, this change aims to allow the Gitaly security repo to kick off the rails test suite on the GitLab security repo.