`bevy_ecs` does not compile if `multi_thread` is enabled but `std` is not

[hukasu]

Bevy version and features

dc14df3

What you did

I'm creating a justfile script that runs clippy on many permutations of features.

What went wrong

cargo clippy -p bevy_ecs --no-default-features --features="multi_threaded" fails with

error[E0665]: `#[derive(Default)]` on enum with no `#[default]`
  --> crates/bevy_ecs/src/schedule/executor/mod.rs:50:25
   |
50 |   #[derive(PartialEq, Eq, Default, Debug, Copy, Clone)]
   |                           ^^^^^^^
51 | / pub enum ExecutorKind {
52 | |     /// Runs the schedule using a single thread.
53 | |     ///
54 | |     /// Useful if you're dealing with a single-threaded environment, saving your threads for
...  |
68 | |     MultiThreaded,
69 | | }
   | |_- this enum needs a unit variant marked with `#[default]`
   |
help: make this unit variant default by placing `#[default]` on it
   |
57 |     #[default] SingleThreaded,
   |     ++++++++++
help: make this unit variant default by placing `#[default]` on it
   |
64 |     #[default] Simple,
   |     ++++++++++

For more information about this error, try `rustc --explain E0665`.
error: could not compile `bevy_ecs` (lib) due to 1 previous error
error: Recipe `bevy_ecs` failed on line 16 with exit code 101

[hukasu]

Found using #21145