Make ensure_runner_queue_value atomic

app/models/ci/runner.rb

@@ -132,6 +132,7 @@ module Ci
 
     def ensure_runner_queue_value
       Gitlab::Redis.with do |redis|
+        redis.multi do
           value = redis.get(runner_queue_key)
           if value == ""
             value = Time.new.inspect
@@ -140,6 +141,7 @@ module Ci
           value
         end
       end
+    end
 
     def is_runner_queue_value_latest?(value)
       ensure_runner_queue_value == value if value.present?