ci: fix build cache (#950)

diff --git a/images/staffeln/Earthfile b/images/staffeln/Earthfile
index 2f60cc3..8cef127 100644
--- a/images/staffeln/Earthfile
+++ b/images/staffeln/Earthfile
@@ -1,6 +1,5 @@
 VERSION 0.7
 
-ARG --global REGISTRY=ghcr.io/vexxhost/atmosphere
 ARG --global PROJECT=staffeln
 ARG --global RELEASE=master
 ARG --global PROJECT_REF=v2.2.3
@@ -15,6 +14,7 @@
 image:
   FROM ../openstack-service+image --RELEASE ${RELEASE} --PROJECT ${PROJECT}
   COPY +build/venv /var/lib/openstack
+  ARG REGISTRY=ghcr.io/vexxhost/atmosphere
   SAVE IMAGE --push \
     ${REGISTRY}/${PROJECT}:${RELEASE} \
     ${REGISTRY}/${PROJECT}:${PROJECT_REF}