ie: replicate pre-datamodel-renderer single-field unique behaviour

The first unique index defined on the field wins. This became visible in
introspection CI after
#3333 was merged.

Arguably, we should instead introspect `@@unique()`s on the model for
extra single-field uniques, but that would be a change.

introspection-engine/datamodel-renderer/src/datamodel/model.rs

@@ -254,6 +254,7 @@ impl<'a> Model<'a> {
         let uniques: HashMap<&str, IndexFieldOptions> = dml_model
             .indices
             .iter()
+            .rev() // replicate existing behaviour on duplicate unique constraints
             .filter(|ix| ix.is_unique())
             .filter(|ix| ix.defined_on_field)
             .map(|ix| {