ci: Experimental legs upload no job-summary artifact, so reports carry no summaries #2078

Found in the CI reports (Discussion #2072, Experimental run 39898485708 at 27288aa93, the #2071 merge commit). The reporter found no step summaries to fold in.

• Artifacts: the run's only artifacts are resource-monitor-graalpy-25.4 and resource-monitor-3.14t. There is no job-summary-*.
• Steps: on both legs, Experimental summary, Name the job summary and Upload the job summary all succeeded.
• So: the upload ran but found no file, because it is if-no-files-found: ignore. Either the summary copy (JOB_SUMMARY_COPY / $RUNNER_TEMP/job-summary.md) is never written on these legs, or it is written to a path the upload doesn't read.

The cause is not known yet; a worker is reading the step logs.