ci: fix build cache (#950)

diff --git a/images/octavia/Earthfile b/images/octavia/Earthfile
index 906e325..c55144c 100644
--- a/images/octavia/Earthfile
+++ b/images/octavia/Earthfile
@@ -1,6 +1,5 @@
 VERSION 0.7
 
-ARG --global REGISTRY=ghcr.io/vexxhost/atmosphere
 ARG --global PROJECT=octavia
 ARG --global RELEASE=2023.2
 ARG --global PROJECT_REF=88d7315a60314e44fcce88ad198ceb3c0c107fe6
@@ -17,6 +16,7 @@
   COPY +build/venv /var/lib/openstack
   DO ../+APT_INSTALL \
     --PACKAGES "isc-dhcp-client openssh-client"
+  ARG REGISTRY=ghcr.io/vexxhost/atmosphere
   SAVE IMAGE --push \
     ${REGISTRY}/${PROJECT}:${RELEASE} \
     ${REGISTRY}/${PROJECT}:${PROJECT_REF}