Configure CNG on dev.gitlab.org to use `buildx-dev` cluster

with buildx-dev cluster completion https://gitlab.com/gitlab-org/distribution/infrastructure/kubernetes-provisioning/-/issues/42+ we need to configure CNG mirror on dev.gitlab.org use buildx-dev for container building.

Things to configure:

• BUILDX_SA_JSON
• BUILDX_GCLOUD_PROJECT
• BUILDX_CLUSTER_NAME
• BUILDX_CLUSTER_ZONE