ci: comment survives a missing baseline, picks the right attempt

Review fixes for the comment workflow. Artifacts are downloaded by name
for the exact attempt that finished, and for the baseline run's latest
attempt, since a rerun keeps earlier attempts' artifacts under the same
run. Every lookup prints nothing instead of failing, so no quattro
baseline yet, an expired one, or a missing boot file shows as "–"
rather than ending the script under set -e before any comment. The
install test's own marker step takes ownership of test-runs first,
which the harness created as root inside its container.

.github/workflows/install-test.yml

@@ -89,7 +89,11 @@ jobs:
       # it needs the PR number, which a workflow_run event omits for forks.
       - name: Record the pull request
         if: always() && github.event_name == 'pull_request'
-        run: mkdir -p test-runs && echo "${{ github.event.pull_request.number }}" > test-runs/pr-number
+        run: |
+          # The harness ran as root in its container and created test-runs.
+          mkdir -p test-runs
+          sudo chown "$(id -u):$(id -g)" test-runs
+          echo "${{ github.event.pull_request.number }}" > test-runs/pr-number
 
       - name: Upload test artifacts
         if: always()