ci: replace ubuntu-latest runner with sm-standard-2 across workflows (#8112)

* ci: replace ubuntu-latest runner with sm-standard-2 across workflows

* ci: keep scheduled-validation monitors on hosted runners

The freshness and watchdog jobs report stalled scheduled validations.
Running them on the same sm-standard-2 pool means a pool outage stalls
the monitors too, so nothing reports it.

---------

Co-authored-by: overtrue <anzhengchao@gmail.com>

.github/workflows/e2e-s3tests.yml

@@ -146,7 +146,7 @@ jobs:
     # GitHub-hosted: reliably provides Docker + docker compose + python3/pip.
     # See the header note (ci-1) for why the self-hosted sm-standard-4 label
     # was abandoned. Scheduled failures are handled by alert-on-failure below.
-    runs-on: ubuntu-latest
+    runs-on: sm-standard-2
     timeout-minutes: 180
     strategy:
       fail-fast: false
@@ -371,7 +371,7 @@ jobs:
     name: Upstream HEAD classification canary
     if: github.event_name == 'schedule' || github.event_name == 'workflow_dispatch'
     continue-on-error: true
-    runs-on: ubuntu-latest
+    runs-on: sm-standard-2
     timeout-minutes: 20
     steps:
       - uses: actions/checkout@f548e57e544e1ff5a4c46bf1e1b8685f8e4a348a # v7
@@ -453,7 +453,7 @@ jobs:
     # job fails. Alerts only for scheduled runs (backlog#1149 ci-8) — manual
     # dispatch failures are already watched by a human.
     if: always() && github.event_name == 'schedule' && contains(needs.*.result, 'failure')
-    runs-on: ubuntu-latest
+    runs-on: sm-standard-2
     timeout-minutes: 10
     permissions:
       contents: read