Lightweight artifacts

Some artifacts are intentionally "Tainted" such that they will never be uploaded and shared with others.

These include for artifacts from workspaced builds, among other cases, possibly artifacts built in non-strict.

For these, it is unimportant to tar the result or ostree commit the result; hardlinking the artifact files directly into a new directory directly in the artifact extract directory should be enough for artifacts we dont need to push.

Fixing this may imply that we need to change the ostree artifact extract directory layout to again match real artifact cache keys.