QA is unable to find an image for RC releases

Similar to #446 (closed), we need to also be able to detect an RC release and use it as the tag for pulling a QA image.

@nolith comments in slack

What I can see here is that we have a 12.6.0-rc42-ee qa image, but not one with the sha. https://dev.gitlab.org/gitlab/omnibus-gitlab/container_registry