Fix issues with asset download up-to-date check with new task output system

Author: lukebemish

src/main/java/dev/lukebemish/taskgraphrunner/runtime/tasks/DownloadAssetsTask.java

@@ -44,7 +44,7 @@ public Map<String, String> outputTypes() {
 
     @Override
     protected boolean upToDate(long lastExecuted, Context context) {
-        var propertiesPath = context.taskOutputPath(this, "properties");
+        var propertiesPath = context.existingTaskOutput(this, "properties");
         var properties = new Properties();
         try (var reader = Files.newBufferedReader(propertiesPath, StandardCharsets.UTF_8)) {
             properties.load(reader);