Fix array_texture example (#7543)

# Objective

- Shader error cause by a missing import.
- `pbr_functions.wgsl` was missing an import for the `ambient_light()` function, as `array_texture` doesn't import it.
- Closes #7542.

## Solution

- Add`#import bevy_pbr::pbr_ambient` into `array_texture`

assets/shaders/array_texture.wgsl

@@ -8,6 +8,7 @@
 #import bevy_pbr::shadows
 #import bevy_pbr::fog
 #import bevy_pbr::pbr_functions
+#import bevy_pbr::pbr_ambient
 
 @group(1) @binding(0)
 var my_array_texture: texture_2d_array<f32>;