Prevent overriding registry location

Part of https://gitlab.developers.cam.ac.uk/uis/devops/iam/activate-account/account-selector/-/issues/14

Found that multi-stage built images were not being pushed out to the artifact registry correctly, and this is believed to resolve that.

Prevents the CI_ARTIFACT_REGISTRY_REPOSITORY environment variable being overriden in the artifact-registry jobs if it was previously set.