Fix datagen task output

src/main/java/net/fabricmc/loom/configuration/FabricApiExtension.java

@@ -208,7 +208,7 @@ public void configureDataGeneration(Action<DataGenerationSettings> action) {
 
 			// Add the output directory as an output allowing the task to be skipped.
 			getProject().getTasks().named("runDatagen", task -> {
-				task.getOutputs().file(outputDirectory);
+				task.getOutputs().dir(outputDirectory);
 			});
 		}
 	}