The buildkite agent supports multiple tags with the same key. This functionality is used to have a [single agent listen on multiple queues](https://buildkite.com/docs/agent/v3/queues#setting-an-agents-queue). However, having the tags be of type `attrsOf str` means that we cannot suport this use case. This commit modifies the type of tags to be `attrsOf (either str (listOf str))` where the list is expanded into multiple tags with the same key. Example: ``` {tags = {queue = ["default", "testing"];};} ``` generates ``` tags="queue=default,queue=testing" ``` in the buildkite agent configuration.