policy/local_chain: return the chain in build_chain()

Makes the code a bit more readable.

gateway/src/apicast/policy/local_chain/local_chain.lua

@@ -28,7 +28,11 @@ local function build_chain(context)
   local proxy = Proxy.new(context.configuration)
 
   context.proxy = context.proxy or proxy
-  context.policy_chain = find_policy_chain(context)
+
+  local policy_chain = find_policy_chain(context)
+  context.policy_chain = policy_chain
+
+  return policy_chain
 end
 
 -- forward all policy methods to the policy chain
@@ -45,9 +49,9 @@ end
 local rewrite = _M.rewrite
 
 function _M:rewrite(context, ...)
-  build_chain(context)
+  local policy_chain = build_chain(context)
 
-  context = LinkedList.readwrite(context, find_policy_chain(context):export())
+  context = LinkedList.readwrite(context, policy_chain:export())
 
   rewrite(self, context, ...)
 end