Fix CI for forks by removing submodule cloning in CI

As far as I could tell there wasn't a reason to clone submodules in CI -- none of the custom frame code was getting used. So the easiest way to get CI working on the fork was just to disable submodule cloning by removing the GIT_SUBMODULE_STRATEGY behaviour.

If this isn't the intended behaviour (i.e. if we actually do need to build the custom frame example in CI) then we can delete this MR.

Also switched from the deprecated only: keyword to using rules:, which probably didn't deserve a MR on its own but figured I'd do it here.