aws-sdk-lambda: DRY up region fetch

don't attempt to determine the region twice within a single method and
simply re-use the result of the single call

lib/new_relic/agent/instrumentation/aws_sdk_lambda/instrumentation.rb

@@ -79,7 +79,7 @@ def generate_segment(action, options = {})
 
       segment = NewRelic::Agent::Tracer.start_external_request_segment(
         library: INSTRUMENTATION_NAME,
-        uri: "https://lambda.#{aws_region || 'unknown-region'}.amazonaws.com",
+        uri: "https://lambda.#{region || 'unknown-region'}.amazonaws.com",
         procedure: action
       )
       segment.name = "External/Lambda/#{action}/#{function}"