Switch from wrapper image to upstream Kaniko

Via CERN Gitlab support:

I've noticed that you are using the wrapper image gitlab-registry.cern.ch/ci-tools/docker-image-builder, as stated in https://gitlab.cern.ch/corryvreckan/corryvreckan/-/blob/master/.gitlab-ci.yml#L415. We have plans for decommissioning the usage of this image in favor of the official image provided by Kaniko in the near future (i.e., gcr.io/kaniko-project/executor:debug, or gcr.io/kaniko-project/executor:v1.12.0-debug if a concrete tag is needed, you can navigate to https://gcr.io/kaniko-project/executor to see the available images). Just by changing the image as stated above things should keep working without any further action.

This MR switches to the upstream Kaniko image. Let's merge and see if nightlies work 😄