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.
