Add Yarn support to omnibus build images
GitLab CE now includes a yarn.lock file to keep our frontend packages locked to a particular version and prevent "works/doesn't work on my machine" type issues. The CI build images have been updated to support this in gitlab-build-images!20 (merged) but the omnibus images need to be updated for this too.
Once this is ready I will open a separate issue/MR within the gitlab-omnibus repo to handle the changes there.
Related: gitlab-ce#27613