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/user_attention/src/main.rs

@@ -4,7 +4,8 @@
 use eframe::{CreationContext, NativeOptions, egui};
 use egui::{Button, CentralPanel, UserAttentionType};
 
-use std::time::{Duration, SystemTime};
+use core::time::Duration;
+use std::time::SystemTime;
 
 fn main() -> eframe::Result {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).