Remove GitLab based caching with buildctl due to performance issues

GitLab's centralize caching system relies on zipping/unzipping cache directories for each job. This does not work well with buildctl is the image blobs and manifests pile up quickly and are often slower to unarchive than they are to build from scratch.

The location of the cache under the project's directory is also an issue, since this becomes part of the build context that is sent to buildkit. If we re-implement this feature using local directory caches, we should locate the directory outside of the project root.