fix: flake.nix under NixOS

[dvtfl]

There are multiple issues with flake.nix.

1. rust-src is missing in rust-toolchain.toml.
   Because of that, Rust Analyzer fails to resolve std, rendering RA pretty much unusable.

(Zed log)

[WARN  project] Language server rust-analyzer-2024-09-02 (id 1) status update: Workspace `~/cosmic/cosmic-comp/Cargo.toml` has sysroot errors: can't load standard library from sysroot
{sysroot_path}
(discovered via `rustc --print sysroot`)
try installing the Rust source the same way you installed rustc

flake.nix imports rust toolchain from it:

cosmic-comp/flake.nix

Line 26 in 52280e9

rust-toolchain = pkgs.rust-bin.fromRustupToolchainFile ./rust-toolchain.toml;

2. When running in Wayland session in NixOS, cosmic-comp fails since libwayland-egl.so is missing in runtime.

2024-09-04T12:46:55.406051Z ERROR backend_winit: panic: thread 'main' panicked at 'Library libwayland-egl.so could not be loaded.': /home/dvtfl/.cargo/registry/src/index.crates.io-6f17d22bba15001f/wayland-sys-0.31.4/src/egl.rs:46
<...>
  26: smithay::backend::winit::init
             at /home/dvtfl/.cargo/git/checkouts/smithay-0141b1fcb9e16729/65c4abf/src/backend/winit/mod.rs:71:5
  27: cosmic_comp::backend::winit::init_backend
             at /home/dvtfl/projects/rust/cosmic/cosmic-comp/src/backend/winit.rs:132:9
  28: cosmic_comp::backend::init_backend_auto
             at /home/dvtfl/projects/rust/cosmic/cosmic-comp/src/backend/mod.rs:35:25
  29: cosmic_comp::main
             at /home/dvtfl/projects/rust/cosmic/cosmic-comp/src/main.rs:108:
<...>

full backtrace
3. When running in X11 session in NixOS, cosmic-comp fails since it fails to instantiate Vulkan and GBM.

INFO cosmic_comp::backend::x11: EGL hardware-acceleration enabled.
WARN cosmic_comp::backend::x11: Failed to instanciate vulkan, falling back to gbm allocator. err=Load(LoadError)
INFO smithay::backend::drm::device::fd: Dropping device: Some("/dev/dri/renderD128")
ERROR cosmic_comp::backend::x11: Failed to create GBM device. err=Os { code: 2, kind: NotFound, message: "No such file or directory" }
WARN cosmic_comp::backend: Initializing X11 Backend failed. err=Failed to create allocator for x11
INFO cosmic_comp::backend: Falling back to winit backend.
INFO backend_winit: smithay::backend::winit: Initializing a winit backend
Error: Failed to initilize winit backend: EventLoopCreation(NotSupported(NotSupportedError))

This PR addresses issues above and additionally reformats flake.nix using nixfmt-rfc-style.

[Drakulix]

thanks!

[dvtfl]

feel free to mention me with Nix related issues :)