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/external_eventloop_async/src/app.rs

@@ -1,6 +1,7 @@
 #![expect(clippy::unwrap_used)] // It's an example
 
-use std::{cell::Cell, io, os::fd::AsRawFd as _, rc::Rc, time::Duration};
+use core::{cell::Cell, time::Duration};
+use std::{io, os::fd::AsRawFd as _, rc::Rc};
 
 use tokio::task::LocalSet;
 use winit::event_loop::{ControlFlow, EventLoop};