CUE: Use enums as map index, produces generic output

Having:
```cue
MyEnum: "a" | "b" | "c"

MyStruct: {
  val: map[MyEnum]bool
}
```

produces (Go):
```go
Val interface{} `json"val"`
```

instead of:
```go
Val map[MyEnum]bool `json:"val"`
```

~This error is in all languages.~ Edit: This is an expected behaviour of cue since its managed as a disjunction, ignoring if all values are strings.