feat: lock down to using the specific rust version

flake.nix

@@ -20,6 +20,7 @@
   }:
     utils.lib.eachDefaultSystem (
       system: let
+        overrides = (builtins.fromTOML (builtins.readFile ./rust-toolchain.toml));
         pkgs = (import nixpkgs) {inherit system;};
         naersk' = pkgs.callPackage naersk {};
         package_name = "skynet_discord_bot";
@@ -62,7 +63,15 @@
 
         # `nix develop`
         devShell = pkgs.mkShell {
-          nativeBuildInputs = with pkgs; [rustc cargo pkg-config openssl rustfmt];
+          nativeBuildInputs = with pkgs; [rustup rustPlatform.bindgenHook pkg-config openssl];
+          # libraries here
+          buildInputs = [ ];
+          RUSTC_VERSION = overrides.toolchain.channel;
+          # https://github.com/rust-lang/rust-bindgen#environment-variables
+          shellHook = ''
+            export PATH="''${CARGO_HOME:-~/.cargo}/bin":"$PATH"
+            export PATH="''${RUSTUP_HOME:-~/.rustup}/toolchains/$RUSTC_VERSION-${pkgs.stdenv.hostPlatform.rust.rustcTarget}/bin":"$PATH"
+          '';
         };
 
         nixosModule = {