add nix flake

flake.nix

@@ -0,0 +1,41 @@
+{
+  description = "GoDaddy DDNS — update A records via REST";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = nixpkgs.legacyPackages.${system};
+      in {
+        packages.default = pkgs.rustPlatform.buildRustPackage {
+          pname = "godaddy_ddns";
+          version = "0.1.0";
+          src = ./.;
+          cargoLock.lockFile = ./Cargo.lock;
+
+          nativeBuildInputs = [ pkgs.pkg-config ];
+          buildInputs = [ pkgs.openssl ]
+            ++ pkgs.lib.optionals pkgs.stdenv.isDarwin [
+              pkgs.darwin.apple_sdk.frameworks.Security
+              pkgs.darwin.apple_sdk.frameworks.SystemConfiguration
+            ];
+
+          doCheck = false;
+        };
+
+        devShells.default = pkgs.mkShell {
+          packages = [
+            pkgs.cargo
+            pkgs.rustc
+            pkgs.rustfmt
+            pkgs.clippy
+            pkgs.pkg-config
+            pkgs.openssl
+          ];
+        };
+      });
+}