ci: make src-cache upload atomic and sweep orphaned temp files
The checkout action's cp of the ~6GB zstd archive directly to the final
path on the cache share is non-atomic; an interrupted copy or a
concurrent reader produces zstd "Read error (39): premature end" on
restore, and the truncated file then satisfies the existence check so
no later run repairs it.
Upload to a run-unique *.tar.upload-<run_id>-<attempt> temp name on the
share and mv to the final path, discarding our temp if a concurrent run
got there first. A new clean-orphaned-cache-uploads workflow removes
temp files older than 4h every 4 hours.
Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Sam Attard <sattard@anthropic.com>
6f1d53ae8f