Don't restrict the scene viewer to loading assets from approved paths. (#18828)

The purpose of the scene viewer is to load arbitrary glTF scenes, so
it's inconvenient if they have to be moved into the Bevy assets
directory first. Thus this patch switches the scene viewer to use
`UnapprovedPathMode::Allow`.

---------

Co-authored-by: François Mockers <francois.mockers@vleue.com>

examples/tools/scene_viewer/main.rs

@@ -10,6 +10,7 @@
 
 use argh::FromArgs;
 use bevy::{
+    asset::UnapprovedPathMode,
     core_pipeline::prepass::{DeferredPrepass, DepthPrepass},
     pbr::DefaultOpaqueRendererMethod,
     prelude::*,
@@ -74,6 +75,8 @@ fn main() {
             })
             .set(AssetPlugin {
                 file_path: std::env::var("CARGO_MANIFEST_DIR").unwrap_or_else(|_| ".".to_string()),
+                // Allow scenes to be loaded from anywhere on disk
+                unapproved_path_mode: UnapprovedPathMode::Allow,
                 ..default()
             }),
         CameraControllerPlugin,