Use our defined rubocop version on CI
Use our defined rubocop version on CI

The default is to use the latest version. That's often fine because we
keep our version up-to-date but it may not always be. It's also great if
builds are re-producible.

lin-d-hop/openfoodnetwork,openfoodfoundation/openfoodnetwork,openfoodfoundation/openfoodnetwork,openfoodfoundation/openfoodnetwork,lin-d-hop/openfoodnetwork,lin-d-hop/openfoodnetwork,mkllnk/openfoodnetwork,lin-d-hop/openfoodnetwork,openfoodfoundation/openfoodnetwork,mkllnk/openfoodnetwork,mkllnk/openfoodnetwork,mkllnk/openfoodnetwork