ci: fix build cache (#950)

diff --git a/images/horizon/Earthfile b/images/horizon/Earthfile
index 3c1cf23..9b77d4e 100644
--- a/images/horizon/Earthfile
+++ b/images/horizon/Earthfile
@@ -1,6 +1,5 @@
 VERSION 0.7
 
-ARG --global REGISTRY=ghcr.io/vexxhost/atmosphere
 ARG --global PROJECT=horizon
 ARG --global RELEASE=2023.2
 ARG --global PROJECT_REF=e650127975265bd398c5ccd0f9fc94456a013e0f
@@ -33,6 +32,7 @@
   COPY +build/venv /var/lib/openstack
   DO ../+APT_INSTALL \
     --PACKAGES "apache2 gettext libapache2-mod-wsgi-py3"
+  ARG REGISTRY=ghcr.io/vexxhost/atmosphere
   SAVE IMAGE --push \
     ${REGISTRY}/${PROJECT}:${RELEASE} \
     ${REGISTRY}/${PROJECT}:${PROJECT_REF}