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/Cargo.toml

@@ -56,8 +56,9 @@ serde = { workspace = true, optional = true }
 
 [dev-dependencies]
 criterion.workspace = true
-egui_kittest = { workspace = true, features = ["wgpu", "snapshot"] }
 egui = { workspace = true, features = ["default_fonts"] }
+egui_kittest = { workspace = true, features = ["wgpu", "snapshot"] }
+mimalloc.workspace = true                                            # for benchmarks
 rand = "0.9"
 
 [[bench]]