vimPlugins/copilot-vim: specify license (unfree)

From [the license][1]:

> GitHub Copilot is offered under the GitHub Terms of Service.
> Copyright (C) 2023 GitHub, Inc. - All Rights Reserved.

[1]: https://github.com/github/copilot.vim/blob/5b19fb001d7f31c4c7c5556d7a97b243bd29f45f/LICENSE.md

pkgs/applications/editors/vim/plugins/overrides.nix

@@ -436,6 +436,8 @@
         --replace "  let node = get(g:, 'copilot_node_command', ''\'''\')" \
                   "  let node = get(g:, 'copilot_node_command', '${nodejs}/bin/node')"
     '';
+
+    meta.license = lib.licenses.unfree;
   };
 
   coq_nvim = super.coq_nvim.overrideAttrs {