ci: fix build cache (#950)

diff --git a/images/heat/Earthfile b/images/heat/Earthfile
index f2eb878..8c3ebcd 100644
--- a/images/heat/Earthfile
+++ b/images/heat/Earthfile
@@ -1,6 +1,5 @@
 VERSION 0.7
 
-ARG --global REGISTRY=ghcr.io/vexxhost/atmosphere
 ARG --global PROJECT=heat
 ARG --global RELEASE=2023.2
 ARG --global PROJECT_REF=d1363cc17646893054f9e8daf40de67699078e7c
@@ -16,6 +15,7 @@
   COPY +build/venv /var/lib/openstack
   DO ../+APT_INSTALL \
     --PACKAGES "curl jq"
+  ARG REGISTRY=ghcr.io/vexxhost/atmosphere
   SAVE IMAGE --push \
     ${REGISTRY}/${PROJECT}:${RELEASE} \
     ${REGISTRY}/${PROJECT}:${PROJECT_REF}