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.

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

.github/workflows/e2e-distributed.yml

@@ -80,7 +80,7 @@ jobs:
     name: Distributed 4-node 4-disk e2e
     # GitHub-hosted VM: loop and tmpfs mounts work here. sm-standard-4 is an
     # ARC pod and rejects both (`mount -o loop` ENOENT, tmpfs "read-only").
-    runs-on: ubuntu-latest
+    runs-on: sm-standard-2
     timeout-minutes: 180
     env:
       FORCE_JAVASCRIPT_ACTIONS_TO_NODE24: "true"
@@ -200,7 +200,7 @@ jobs:
     name: Alert on scheduled failure
     needs: [distributed]
     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