Add flake.nix

Add a new flake.nix for this repo, having extracted it from
nixos-configs. Add packages.nix copied from nixos-configs, as I
accidentally removed it while filtering to create this repo.

pkgs/qrclip/default.nix

@@ -1,6 +1,12 @@
-{ pkgs }:
-let zbar = pkgs.mypkgs.zbar.override { enableVideo = false; };
-in pkgs.writeShellScriptBin "qrclip" ''
-  set -eo pipefail
-  ${pkgs.pngpaste}/bin/pngpaste - | ${zbar}/bin/zbarimg --raw -q1 -
-''
+{
+  pkgs,
+  lib,
+  stdenv,
+}: let
+  zbar = pkgs.mypkgs.zbar.override {enableVideo = false;};
+in
+  (pkgs.writeShellScriptBin "qrclip" ''
+    set -eo pipefail
+    ${pkgs.pngpaste}/bin/pngpaste - | ${zbar}/bin/zbarimg --raw -q1 -
+  '')
+  // {meta.platforms = lib.platforms.darwin;}