switch to fenix to manage rust toolchain in nix

flake.nix

@@ -1,12 +1,30 @@
 {
   inputs = {
     nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+
+    fenix = {
+      url = "github:nix-community/fenix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
   };
 
   outputs =
-    { self, nixpkgs }:
+    {
+      self,
+      nixpkgs,
+      fenix,
+    }:
     let
       pkgs = nixpkgs.legacyPackages.x86_64-linux;
+      rustPkgs = fenix.packages.x86_64-linux.default;
+
+      rustPlatform = pkgs.makeRustPlatform {
+        inherit (rustPkgs)
+          cargo
+          rustc
+          ;
+      };
+
       nativeBuildInputs = with pkgs; [
         pkg-config
         m4
@@ -29,7 +47,7 @@
       };
 
       devShells.x86_64-linux.default = mkShell {
-        packages = [
+        packages = with rustPkgs; [
           cargo
           clippy
           rustc