fix(ci): merge all duration slices, not one

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.

.github/workflows/tests.yml

@@ -155,18 +155,22 @@ jobs:
     timeout-minutes: 10
     steps:
       - name: Download all slice durations
+        # Each slice uploads the same file name (test_durations.json).
+        # With merge-multiple, the parallel downloads write to one path.
+        # This causes two problems: a race can write two JSON documents
+        # into one file, and the last write erases the other slices.
+        # Without merge-multiple, each artifact gets its own directory.
         uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8.0.1
         with:
           pattern: test-durations-slice-*
           path: durations
-          merge-multiple: true
 
       - name: Merge into single durations file
         run: |
           python3 -c "
           import json, glob, os
           merged = {}
-          for f in glob.glob('durations/*test_durations.json'):
+          for f in glob.glob('durations/*/test_durations.json'):
             with open(f) as fh:
               merged.update(json.load(fh))
           with open('test_durations.json', 'w') as fh: