[gitlab-operator-build-base] Point directly to kubebuilder release artifact URL

Points directly to the kubebuilder release artifact URL in GitHub, rather than going through go.kubebuilder.io. Its redirection, along with the artifact names between versions, has changed and become unstable. This approach gives us a more clear target for the download and will make it easier to patch in the future when newer versions have different filename conventions (see https://github.com/kubernetes-sigs/kubebuilder/releases).

Closes #89 (closed)