`UseBuiltInTrxReporter` should default to true

We don't document this configuration option anywhere, yet it is essential to making sure that MNTR output is readable in environments like Azure DevOps:

```json
{
  "useBuiltInTrxReporter":true
}
```

We hackily inject it into the main Akka.NET project via FAKE today:

https://github.com/akkadotnet/akka.net/blob/5d8965bc82b4b37e67679a03f67b18e22891dc2d/build.fsx#L317-L319