CI: allow failure in new jobs 'handle_artifacts' and 'report-image-diff'

It seems that these jobs are not yet completly errorsafe and we could
ignore the failure without changing the build artifacts.

manifest-integration-jobs.yml

@@ -71,6 +71,7 @@ report-image-diff:
     - if: $CI_MERGE_REQUEST_LABELS =~ /skip build/
       when: never
     - if: $CI_MERGE_REQUEST_IID
+  allow_failure: true
   script:
     - cd ${CI_PROJECT_DIR}
     - .gitlab-ci/scripts/report_image_diff.py

manifest-pipeline.yml

@@ -145,6 +145,7 @@ handle_artifacts:
   stage: artifacts
   needs: ["build:merge_request"]
   timeout: 1h
+  allow_failure: true
   script:
     - cd ${CI_PROJECT_DIR}
     - .gitlab-ci/scripts/handle_artifacts.py