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.

Works in APSQ already: allpix-squared/allpix-squared!1050 (merged)