pinwheel: Use lib.genAttrs to create dev shells for multiple systems

hosts/pinwheel/modules/skal/flake-template.nix

@@ -7,12 +7,18 @@
 
   outputs = { nixpkgs, ... }:
     let
-      system = "x86_64-linux";
+      systems = [ "x86_64-linux" ];
+    in
+      {
+        devShells = nixpkgs.lib.genAttrs systems (system:
+          let
             pkgs = nixpkgs.legacyPackages.${system};
           in
             {
-        devShells."${system}".default = pkgs.mkShell {
+              default = pkgs.mkShell {
                 packages = [];
               };
+            }
+        );
       };
 }