ci: traces tests flaky at downloading
We have two characteristic messages from traces flakes when downloading apparently fails:
warning: unexpected end of file while reading trace
https://mesa.pages.freedesktop.org/-/mesa/-/jobs/15241417/artifacts/results/summary/results/trace@freedreno-a630@0ad@0ad.trace.html
error: expected digit, found '-'
https://mesa.pages.freedesktop.org/-/mesa/-/jobs/15241417/artifacts/results/summary/results/trace@freedreno-a630@valve@half-life-2-v2.trace.html
(note that I've checked around when this happens, and we have tons of disk space avaialable)
We need some logging for what happened, probably a retry if download fails, and to consider inability to download successfully as a failure instead of trying to replay and diff images of the broken download.