[cr-buildbucket.cfg] Switch swarm-ci builders to use the swarming/staging recipe

These builders do not use the chromium recipe.

Bug: 868525
Change-Id: Iac4a05d46397e70a25e47ca3a27a27300b32cff9
Reviewed-on: https://chromium-review.googlesource.com/1153297Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Ryan Tseng <hinoka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578817}

infra/config/global/cr-buildbucket.cfg

@@ -424,6 +424,7 @@ builder_mixins {
 builder_mixins {
   name: "swarm-ci"
   recipe {
+    name: "swarming/staging"
     properties: "mastername:chromium.swarm"
   }
 }