[cr-buildbucket] Allow luci-migration to bump build number

Change-Id: I0c0a97e78bc11adc93a3746019b4cca3ec8651b5
diff --git a/tools/luci/config/cr-buildbucket.cfg b/tools/luci/config/cr-buildbucket.cfg
index 0f0fbcc..8c0125a 100644
--- a/tools/luci/config/cr-buildbucket.cfg
+++ b/tools/luci/config/cr-buildbucket.cfg
@@ -19,6 +19,10 @@
     role: SCHEDULER
     identity: "luci-scheduler@appspot.gserviceaccount.com"
   }
+  acls {
+    role: WRITER
+    identity: "luci-migration@appspot.gserviceaccount.com"
+  }
 }
 
 buckets {