Add atmosphere_image_overrides options

This is for ability to provide override values for atmosphere_images
diff --git a/Earthfile b/Earthfile
index efd12fa..197fc0d 100644
--- a/Earthfile
+++ b/Earthfile
@@ -60,10 +60,10 @@
 
 pin-images:
   FROM +build.venv.dev
-  COPY roles/defaults/defaults/main.yml /defaults.yml
+  COPY roles/defaults/vars/main.yml /defaults.yml
   COPY build/pin-images.py /usr/local/bin/pin-images
   RUN /usr/local/bin/pin-images /defaults.yml /pinned.yml
-  SAVE ARTIFACT /pinned.yml AS LOCAL roles/defaults/defaults/main.yml
+  SAVE ARTIFACT /pinned.yml AS LOCAL roles/defaults/vars/main.yml
 
 gh:
   FROM alpine:3