pkgs: make system-dependent packages optional

This helps reduce failures in ``nix flake check .``.

pkgs/default.nix

@@ -5,7 +5,8 @@ in
 {
   awesome-lint = cp ./all/awesome-lint { };
   crossover = cp ./all/crossover.nix { };
-  developercolorpicker = cp ./darwin/developercolorpicker.nix { };
   doctoc = cp ./all/doctoc.nix { };
   ultisnips-vscode = cp ./all/ultisnips-vscode.nix { };
+} // pkgs.lib.optionalAttrs pkgs.stdenv.isLinux { } // pkgs.lib.optionalAttrs pkgs.stdenv.isDarwin {
+  developercolorpicker = cp ./darwin/developercolorpicker.nix { };
 }