Fix #134920: File Output writes frame for single render

The File Output node always appends the frame number even if the render
is not an animation. This patch makes it such that the frame number is
only written if the render is an animation. The user can use a frame
variable to manually append the frame number if needed.

The command line rendering interface already uses animation rendering in
all cases, so it should not be affected. However, rendering using the
render.render() operator with animation=False will see the behavior
change, however, setting animation=False and start_frame and end_frame
to the same frame number should be sufficient to restore the old
behavior.

Pull Request: https://projects.blender.org/blender/blender/pulls/141209

tests/python/compositor_file_output_tests.py

@@ -209,7 +209,7 @@ class FileOutputTest(unittest.TestCase):
         # Set output directory for all existing file output nodes.
         set_directory(bpy.data.scenes[0].compositing_node_group, f'{curr_out_dir}/')
         bpy.data.scenes[0].render.compositor_device = f'{self.execution_device}'
-        bpy.ops.render.render()
+        bpy.ops.render.render(animation=True, start_frame=1, end_frame=1)
 
 
 if __name__ == "__main__":