Add synthetic properties to JS engine for exclusion

This fixes an issue where inputs would be declared as new variables, causing a "miss" when trying to read the input.

src/modules/Elsa.JavaScript/Handlers/ConfigureEngineWithVariables.cs

@@ -87,6 +87,11 @@ public class ConfigureEngineWithVariables(IOptions<JintOptions> options) : INoti
                     yield return inputDefinition.Name;
             }
 
+            foreach (var syntheticProperty in activityExecutionContext.Activity.SyntheticProperties)
+            {
+                yield return syntheticProperty.Key;
+            }
+
             activityExecutionContext = activityExecutionContext.ParentActivityExecutionContext;
         }
     }