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_kittest/src/builder.rs

@@ -2,8 +2,8 @@ use crate::app_kind::AppKind;
 #[cfg(feature = "eframe")]
 use crate::app_kind::AppKindEframe;
 use crate::{Harness, LazyRenderer, TestRenderer};
+use core::marker::PhantomData;
 use egui::{Pos2, Rect, Vec2};
-use std::marker::PhantomData;
 
 /// Builder for [`Harness`].
 #[must_use]