Fix multiarch step extends

Signed-off-by: Federico Busetti <[email protected]>

Author: febus982

.gitlab_ci/build.yml

@@ -53,4 +53,4 @@ aggregate-http-manifests:
     - if: $CI_COMMIT_REF_NAME == $CI_DEFAULT_BRANCH
     - if: $CI_PIPELINE_SOURCE == "merge_request_event"
   extends:
-    - .build-and-push-gitlab
+    - .multiarch-manifest-gitlab