bst is still pushing artifacts it just have pulled

Background

See, for example, https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/jobs/69952759

I think this was already solved with #233 (closed) or with #325 (closed), opening another issue in case this is a new issue

Task description

Check artifact exist in the server before try to push?

Acceptance Criteria

bst should not push artifacts to the cache server if they are already there