fix(ci): give the chain staleness probe a token that can read auto-testing (#8041)

resolve_functional_candidate.py probes rustfs/auto-testing (private) to
age the pinned functional-script revision and fall back to main HEAD
after 24h. The prepare step passed github.token, which cannot see the
private repo, so every nightly chain logged

  staleness probe failed (...exit status 1.); keeping pinned revision

and replayed the 09-14 harness. On the 09-20 nightly that harness died
on the dpkg conffile prompt in all 12 lanes (see rustfs/auto-testing#97)
because the --force-confold and other fixes never reached the chain.

Use PF_TESTING_GH_TOKEN - already required by the other steps in this
workflow - so the probe can actually run and the >24h fallback works.

.github/workflows/rustfs-functional-chain.yml

@@ -54,7 +54,10 @@ jobs:
       - name: Resolve published candidate
         id: candidate
         env:
-          GH_TOKEN: ${{ github.token }}
+          # The staleness probe in resolve_functional_candidate.py reads the
+          # private rustfs/auto-testing repo; github.token cannot, and a probe
+          # failure silently keeps a stale testing_sha pinned for the chain.
+          GH_TOKEN: ${{ secrets.PF_TESTING_GH_TOKEN }}
           BUILD_RUN_ID: ${{ inputs.build_run_id }}
           BUILD_RUN_ATTEMPT: ${{ inputs.build_run_attempt }}
           CHAIN_OUTPUT: ${{ runner.temp }}/chain-candidate.json