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

examples/multiple_viewports/src/main.rs

@@ -1,10 +1,8 @@
 #![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
 #![expect(rustdoc::missing_crate_level_docs)] // it's an example
 
-use std::sync::{
-    Arc,
-    atomic::{AtomicBool, Ordering},
-};
+use core::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Arc;
 
 use eframe::egui;