Make UIO results work for 2.12

[vpavkin]

For some reason, this implicit doesn't get picked up on 2.12 when trying to resolve Marshaller for zio.UIO[String] value.

The test is here, and I had to replace UIO with Task to make it compile. For UIO I get

ZIOInteropSpec.scala:21:18: diverging implicit expansion for type 
akka.http.scaladsl.marshalling.Marshaller[A,akka.http.scaladsl.model.HttpResponse] 
starting with method zioSupportIOMarshaller in trait ZIOSupportInstances2

Which indicates, that a specific UIO instance with higher priority doesn't get picked up.

On 2.13 it works fine.