Fix build subtarget compatibility

Certain versions of Unity do not have a value for `NoSubtarget`. Using `default` will be more robust against churn in this area of Unity's API.

dist/default-build-script/Assets/Editor/UnityBuilderAction/Builder.cs

@@ -33,7 +33,7 @@ namespace UnityBuilderAction
       // Determine subtarget
       StandaloneBuildSubtarget buildSubtarget;
       if (!options.TryGetValue("standaloneBuildSubtarget", out var subtargetValue) || !Enum.TryParse(subtargetValue, out buildSubtarget)) {
-        buildSubtarget = StandaloneBuildSubtarget.NoSubtarget;
+        buildSubtarget = default;
       }
 #endif