ci: trace diffs not visible
This has been an issue for a while, but I don't remember what the resolution was. Sometimes job artifacts don't display the reference or diff images, like in https://mesa.pages.freedesktop.org/-/mesa/-/jobs/49566198/artifacts/results/summary/results/trace@gl-zink-anv-tgl@gputest@pixmark-piano-v2.trace.html