Lightmap is not updating

## Bevy version

0.16.1

## \[Optional\] Relevant system information

```ignore
AdapterInfo { name: "NVIDIA RTX A3000 Laptop GPU", vendor: 4318, device: 9400, device_type: DiscreteGpu, driver: "NVIDIA", driver_info: "566.24", backend: Vulkan }
```

## What you did

I tried to run this code:

```rust
use bevy::{
asset::RenderAssetUsages,
pbr::Lightmap,
prelude::*,
render::render_resource::{Extent3d, TextureDimension, TextureFormat},
};

fn main() {
App::new()
.add_plugins(DefaultPlugins)
.add_systems(Startup, setup)
.add_systems(Update, update_lightmap)
.run();
}

fn setup(
mut commands: Commands,
mut meshes: ResMut>,
mut materials: ResMut>,
mut images: ResMut>,
) {
let mut mesh: Mesh = Plane3d::new(Vec3::Y, 5.0 * Vec2::ONE).into();
let uv_0 = mesh.attribute(Mesh::ATTRIBUTE_UV_0).unwrap().clone();
mesh.insert_attribute(Mesh::ATTRIBUTE_UV_1, uv_0);

let image = Image::new_fill(
Extent3d {
width: 1,
height: 1,
..Default::default()
},
TextureDimension::D2,
&[255, 255, 255, 255],
TextureFormat::bevy_default(),
RenderAssetUsages::all(),
);

commands.spawn((
Mesh3d(meshes.add(mesh)),
MeshMaterial3d(materials.add(StandardMaterial {
lightmap_exposure: 250.0,
..Default::default()
})),
Transform::default(),
Lightmap {
image: images.add(image),
..Default::default()
},
));

commands.spawn((
Camera3d::default(),
Transform::from_xyz(-2.5, 4.5, 9.0).looking_at(Vec3::ZERO, Vec3::Y),
));
}

fn update_lightmap(
lightmap: Single<&Lightmap>,
time: Res,
mut images: ResMut>,
) {
let image = images.get_mut(&lightmap.image).unwrap();
let brightness = time.elapsed_secs().sin() * 0.5 + 0.5;
image
.set_color_at(0, 0, Color::srgba(brightness, brightness, brightness, 1.0))
.unwrap();
}
```

## What went wrong

Plane lighting isn't updating

Start by running the reproduction in the issue and inspect the Lightmap component, setup function, and update_lightmap system. Trace how the image asset is consumed by the plane's lighting, then verify that changing the image makes the plane lighting update over time.

Written by the indexing model from the issue text.