Each test slice uploads an artifact with the same file name,
test_durations.json. The save-durations job downloaded the 12
artifacts with merge-multiple, so all extractions wrote to one
path in parallel. This caused two faults:
- A race between two extractions wrote two JSON documents into
one file. The merge step then failed with 'JSONDecodeError:
Extra data' (run 31382130252).
- On green runs, the last write erased the other 11 slices. The
merged cache held ~230 of ~2760 file durations.
Remove merge-multiple so each artifact extracts into its own
directory, and point the glob at durations/*/test_durations.json.
A local merge of the 12 real artifacts from the failed run gives
2761 durations.
03da1606bc