Investigate AWS "Dead Letter" SQS queue

[rmcgibbo]

https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/sqs-dead-letter-queues.html

The idea would be that rather an immediately marking each message as processed, we could set a maxReceiveCount and use a dead letter queue.

One situation where I can imagine something like this being valuable is when the nixpkgs master branch is temporarily broken and won't evaluate, and we might as well retry failing PRs after some delay.