ci: improve dogfooding by using the CI built images directly

.gitlab-ci.yml

@@ -1,10 +1,13 @@
 include:
   - component: $CI_SERVER_FQDN/$CI_PROJECT_PATH/nix-gitlab-ci@$CI_COMMIT_SHA
+    inputs:
+      image_tag: $CI_COMMIT_SHORT_SHA
 stages:
+  - build-images
   - build
   - trigger
 build:image:
-  stage: build
+  stage: build-images
   parallel:
     matrix:
       - VARIANT: ["", "-cachix", "-attic"]