{nixos/}scx: add scx_mitosis

[JohnRTitor]

:fish: What?

1. Looks like there is another rust scheduler we can add https://github.com/sched-ext/scx/tree/main/scheds/rust/scx_mitosis

:fishing_pole_and_fish: Why?

• (1) Why not?

:fish_cake: Pending

• [ ] Fix build failure

:whale: Extras

• I like chocolate cake.