Use mimalloc for benchmarks (#7029)

`mimalloc` is a _much_ faster allocator, especially important when doing
a lot of small allocations (which egui does).

We use `mimalloc` in Rerun, and I recommend everyone to use it.

## The difference it makes

![image](https://github.com/user-attachments/assets/b22e0025-bc5e-4b3c-94e0-74ce46e86f85)

crates/egui_demo_lib/benches/benchmark.rs

@@ -8,6 +8,9 @@ use egui::{Button, Id, RichText, TextureId, Ui, UiBuilder, Vec2};
 use egui_demo_lib::LOREM_IPSUM_LONG;
 use rand::Rng as _;
 
+#[global_allocator]
+static GLOBAL: mimalloc::MiMalloc = mimalloc::MiMalloc; // Much faster allocator
+
 /// Each iteration should be called in their own `Ui` with an intentional id clash,
 /// to prevent the Context from building a massive map of `WidgetRects` (which would slow the test,
 /// causing unreliable results).