fix(ci): make timings report fork-safe (missed by #66577)

#66373 swapped GITHUB_TOKEN -> AUTOFIX_BOT_PAT across the workflows and
#66577 restored the `|| github.token` fork fallback for detect-changes and
the label gates -- but it missed the ci-timings "Collect timings and
generate report" step, which still passes a bare AUTOFIX_BOT_PAT. On fork
PRs that PAT is empty, so timings_report.py hard-fails at
expect_env("GITHUB_TOKEN") before it can reach its own "degraded run must
never redden the PR" soft-fail path. Every fork PR gets a red run from this
advisory job (e.g. #66573).

- ci.yml: apply the same `secrets.AUTOFIX_BOT_PAT || github.token` fallback
  to the timings step. github.token has `actions: read`, enough to read the
  run's job/step durations on forks.
- timings_report.py: treat a missing/empty GITHUB_TOKEN as a degraded run
  (TimingsUnavailable) instead of a hard ValueError, so this whole class of
  failure can never redden a PR again even if a future workflow drops the
  token. Still writes no JSON, so no empty baseline is ever cached.

.github/workflows/ci.yml

@@ -226,7 +226,10 @@ jobs:
 
       - name: Collect timings and generate report
         env:
-          GITHUB_TOKEN: ${{ secrets.AUTOFIX_BOT_PAT }}
+          # Forks get no repo secrets (AUTOFIX_BOT_PAT is empty); fall back to
+          # the built-in read-only token so the timings API read still works
+          # there instead of hard-failing this advisory job on every fork PR.
+          GITHUB_TOKEN: ${{ secrets.AUTOFIX_BOT_PAT || github.token }}
         run: |
           python3 scripts/ci/timings_report.py \
             --baseline ci-timings-baseline.json \