coreutils?

builder.sh

@@ -1,2 +1,3 @@
+export PATH="$coreutils/bin"
 mkdir $out
 cp *\.{png,webp} $out

flake.nix

@@ -2,13 +2,26 @@
   inputs.nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
 
   outputs =
-    { nixpkgs, ... }: {
-      packages."x86_64-linux".default = derivation
-        {
-          name = "backgrounds";
-          builder = "${nixpkgs.bash}/bin/bash";
-          args = [ ./builder.sh ];
-          system = builtins.currentSystem;
-        };
+    { nixpkgs, ... }:
+    let
+      systems = [ "aarch64-linux" "x86_64-linux" ];
+      forAllSystems =
+        function:
+        nixpkgs.lib.genAttrs systems (
+          system: function nixpkgs.legacyPackages.${system} system
+        );
+    in
+    {
+      packages = forAllSystems (pkgs: system: rec {
+        default = backgrounds;
+        backgrounds = derivation
+          {
+            inherit system;
+            inherit (pkgs) coreutils;
+            name = "backgrounds";
+            builder = "${pkgs.bash}/bin/bash";
+            args = [ ./builder.sh ];
+          };
+      });
     };
 }