Add BUILDER_IMAGE_REVISION to cache keys
Add BUILDER_IMAGE_REVISION to cache keys

If something changes in the builder, we need to be aware of this in the
depenedent builds, and not use cached artefacts generated by eariler
builders.

Here we thread BUILDER_IMAGE_REVISION through the cache keys that
previous just used the job name/branch caches.

From the investigation of #3032

gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab