added wrapper

nix/wrapper/default.nix

@@ -0,0 +1,14 @@
+{ pkgs, ... }: pkgs.stdenv.mkDerivation {
+  name = "aoc2021";
+  buildInputs = [ pkgs.haskellPackages.aoc2021-programs ];
+  src = ../../.;
+  installPhase = ''
+    mkdir -p $out/{bin,files}
+    cat > $out/bin/aoc2021 << EOF
+    #!/bin/bash
+    cat $out/files/\''${1}-input.txt | ${pkgs.haskellPackages.aoc2021-programs}/bin/aoc2021-\''${1}
+    EOF
+    chmod +x $out/bin/aoc2021
+    find code/ -type f | grep -i input.txt$ | xargs -i cp {} $out/files
+  '';
+}