Make it possible to trigger gitlab-org/gitlab downstream pipeline

In 7f4aba35 I've attempted to make it possible to trigger a gitlab-org/gitlab pipeline with the GITALY_SERVER_VERSION of the head commit of an MR. But as discussed this isn't that easy:

Given that this looks like it'll trigger a pipeline for the default branch, we don't need to pass FORCE_GITLAB_CI, because in the workflow defined in .gitlab-ci.yml, we do allow for the default branch: https://gitlab.com/gitlab-org/gitlab/-/blob/cad24f511126a6b1d9510dda2527b3e1fdc666ae/.gitlab-ci.yml#L69

- if: '$CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH'
  variables:
    CREATE_INCIDENT_FOR_PIPELINE_FAILURE: "true"
    NOTIFY_PIPELINE_FAILURE_CHANNEL: "master-broken"

That said, it's still better to pass FORCE_GITLAB_CI because here we can see that above will enable NOTIFY_PIPELINE_FAILURE_CHANNEL which we don't want.

If we pass FORCE_GITLAB_CI like we did here, it'll stop on the top which doesn't set additional variables.

This is slightly related to why I said gitlab!102881 (merged) will not stop this from happening. That merge request is adding if-default-refs to various conditions, which also includes FORCE_GITLAB_CI: https://gitlab.com/gitlab-org/gitlab/-/blob/cad24f511126a6b1d9510dda2527b3e1fdc666ae/.gitlab/ci/rules.gitlab-ci.yml#L24

So in short, whenever FORCE_GITLAB_CI is set, it will just run a lot of things, including things we want and we don't want :/

This is not ideal in my opinion, but that's what it is right now and I am not sure how to improve this while keep everything backward compatible. I think we need to improve this and ponder more, but not something we can improve quickly.

And there's also another problem:

Given that it is triggering a pipeline on the default branch (master), which is protected, you need to be a GitLab maintainer to trigger it.

This is the reason why we're triggering QA on a mirror project: https://gitlab.com/gitlab-org/build/omnibus-gitlab-mirror

Where we can give wider permissions. Perhaps we need to find a workaround for this.