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/epaint/benches/benchmark.rs

@@ -5,6 +5,9 @@ use epaint::{
     TessellationOptions, Tessellator, TextureAtlas, Vec2,
 };
 
+#[global_allocator]
+static GLOBAL: mimalloc::MiMalloc = mimalloc::MiMalloc; // Much faster allocator
+
 fn single_dashed_lines(c: &mut Criterion) {
     c.bench_function("single_dashed_lines", move |b| {
         b.iter(|| {