use experimental uutils cp

There is an effort to integrate `nushell` and `uutils/coreutils` and
this `ucp` command is the first step. This aliases to the new command
for testing and will probably need to be removed for the next release.

home-manager/applications/shells/nushell/default.nix

@@ -11,6 +11,7 @@
  cat = "bat";
  tree = "broot";
  vim = "nvim";
+ cp = "ucp";
  };
  extraConfig = ''
  source ${inputs.nu-scripts}/modules/nix/nix.nu