Enable the clippy::std_instead_of_core lint (#8394)

Prefer `core::` over `std::` where either work

* Part of https://github.com/emilk/egui/issues/5735

crates/egui_demo_app/src/apps/custom3d_wgpu.rs

@@ -1,6 +1,6 @@
 #![expect(clippy::unwrap_used)] // TODO(emilk): avoid unwraps
 
-use std::num::NonZeroU64;
+use core::num::NonZeroU64;
 
 use eframe::{
     egui_wgpu::wgpu::util::DeviceExt as _,