[buildbucket] make task force WRITER

Make luci-task-force@google.com group a WRITER of luci.chromium.ci
so that they can bump next build number.

R=vadimsh@chromium.org

No-Try: true
Change-Id: Ice4dfb8e2f9910a8ca42f2b4dfe753d161d0f09c
Reviewed-on: https://chromium-review.googlesource.com/963228
Commit-Queue: Nodir Turakulov <nodir@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543159}

infra/config/global/cr-buildbucket.cfg

@@ -38,6 +38,11 @@ acl_sets {
     # Support builder triggering other builders in the same bucket.
     identity: "chromium-ci-builder@chops-service-accounts.iam.gserviceaccount.com"
   }
+  acls {
+    # Allow task force to bump next build number.
+    role: WRITER
+    group: "luci-task-force@google.com"
+  }
 }
 
 acl_sets {