fix: make flake work for aarch64 and darwin

flake.nix

@@ -1,13 +1,42 @@
 {
-  description = "Repo RS";
+  description = "repo-rs";
 
   inputs = {
-    nixpkgs.url = "github:nixos/nixpkgs?ref=nixos-unstable";
+    nixpkgs.url = "github:nixos/nixpkgs?ref=nixpkgs-unstable";
   };
 
-  outputs = { self, nixpkgs }: {
-    devShells.x86_64-linux.default = import ./shell.nix { pkgs = nixpkgs.legacyPackages.x86_64-linux; };
-    packages.x86_64-linux.repo-rs = nixpkgs.legacyPackages.x86_64-linux.callPackage ./. { };
-    packages.x86_64-linux.default = self.packages.x86_64-linux.repo-rs;
-  };
+  outputs =
+    { self, nixpkgs }:
+    let
+      systems = [
+        "aarch64-darwin"
+        "aarch64-linux"
+        "x86_64-darwin"
+        "x86_64-linux"
+      ];
+      eachSystem =
+        f:
+        nixpkgs.lib.genAttrs systems (
+          system:
+          f rec {
+            inherit system;
+            pkgs = nixpkgs.legacyPackages.${system};
+          }
+        );
+    in
+    {
+      devShells = eachSystem (
+        { pkgs, ... }:
+        {
+          default = import ./shell.nix { inherit pkgs; };
+        }
+      );
+      packages = eachSystem (
+        { pkgs, system, ... }:
+        {
+          repo-rs = pkgs.callPackage ./. { };
+          default = self.packages.${system}.repo-rs;
+        }
+      );
+    };
 }