src dir?

builder.sh

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

flake.nix

@@ -21,6 +21,7 @@
             name = "backgrounds";
             builder = "${pkgs.bash}/bin/bash";
             args = [ ./builder.sh ];
+            src = ./.;
           };
       });
     };