Fix failure reason for dropped builds on CI quota exhaustion

What does this MR do?

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/323486 and #326709 (comment 585367787)

We drop the builds when the quota is exceeded, but we do it with the wrong failure reason because the instance_runners array is empty:

• https://gitlab.com/gitlab-org/gitlab/-/blob/0faaa6a9add20fe2caba46911497f8b607f52dc8/app/services/ci/pipeline_creation/drop_not_runnable_builds_service.rb#L31-32
• https://gitlab.com/gitlab-org/gitlab/-/blob/0faaa6a9add20fe2caba46911497f8b607f52dc8/app/models/project.rb#L1746
• https://gitlab.com/gitlab-org/gitlab/-/blob/0faaa6a9add20fe2caba46911497f8b607f52dc8/app/models/project.rb#L1738
• https://gitlab.com/gitlab-org/gitlab/-/blob/0faaa6a9add20fe2caba46911497f8b607f52dc8/ee/app/models/ee/project.rb#L361-363

And on top of this we were dropping pipelines for public projects that are part of namespaces with used up CI minutes.

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