Make VERTEX_COLORS usable in prepass shader, if available (#10341)

# Objective

I was working with forward rendering prepass fragment shaders and ran
into an issue of not being able to access vertex colors in the prepass.
I was able to access vertex colors in regular fragment shaders as well
as in deferred shaders.

## Solution

It seems like this `if` was nested unintentionally as moving it outside
of the `deferred` block works.

---

## Changelog

Enable vertex colors in forward rendering prepass fragment shaders

crates/bevy_pbr/src/prepass/mod.rs

@@ -431,11 +431,11 @@ where
 
         if key.mesh_key.contains(MeshPipelineKey::DEFERRED_PREPASS) {
             shader_defs.push("DEFERRED_PREPASS".into());
+        }
 
-            if layout.contains(Mesh::ATTRIBUTE_COLOR) {
-                shader_defs.push("VERTEX_COLORS".into());
-                vertex_attributes.push(Mesh::ATTRIBUTE_COLOR.at_shader_location(6));
-            }
+        if layout.contains(Mesh::ATTRIBUTE_COLOR) {
+            shader_defs.push("VERTEX_COLORS".into());
+            vertex_attributes.push(Mesh::ATTRIBUTE_COLOR.at_shader_location(6));
         }
 
         if key