refactor(nixos): inline module enable conditions

configs/nixos/modules/shell/rust.nix

@@ -6,7 +6,6 @@
 }:
 # Rust tools for the primary user's shell environment.
 let
-  cfg = config.modules.shell.rust; # NOLINT(let_order)
   toolchain = pkgs.symlinkJoin {
     name = "rust-toolchain";
     paths = with pkgs; [
@@ -23,7 +22,7 @@ in
   options.modules.shell.rust.enable =
     lib.mkEnableOption "Rust tools for the primary user's shell environment";
 
-  config = lib.mkIf cfg.enable {
+  config = lib.mkIf config.modules.shell.rust.enable {
     home-manager.users.${user} = {
       home.sessionVariables.RUST_TOOLCHAIN_ROOT = "${toolchain}";