ci: fix build cache (#950)

diff --git a/images/placement/Earthfile b/images/placement/Earthfile
index e843c08..5719453 100644
--- a/images/placement/Earthfile
+++ b/images/placement/Earthfile
@@ -1,6 +1,5 @@
 VERSION 0.7
 
-ARG --global REGISTRY=ghcr.io/vexxhost/atmosphere
 ARG --global PROJECT=placement
 ARG --global RELEASE=2023.2
 ARG --global PROJECT_REF=a361622d749d3b24aad638ec1b03a7d7124a87b3
@@ -14,6 +13,7 @@
 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}