Motivation for this change

Some days ago I packaged xptemplate for VIM.
However according to the docs the recommended approach to use custom snippets is to copy them into the personal/ directory (https://github.com/drmingdrmer/xptemplate#extend-xptemplate-write-new-snippets)

The easiest approach is to override the plugin and add a postInstall script, however until now it wasn't possible to override VIM plugins.

I confirmed the change using the following expressions for a test VM:

{
  vim = { pkgs, ... }: {
    environment.systemPackages = let
      vim = with pkgs; vim_configurable.customize {
        name = "xptemplatetest";
        vimrcConfig.vam.knownPlugins = vimPlugins;
        vimrcConfig.vam.pluginDictionaries = [
          { name = "xptemplate"; }
        ];
      };
    in [ vim ];

    nixpkgs.config.packageOverrides = super: {
      vimPlugins = super.vimPlugins // {
        xptemplate = super.vimPlugins.xptemplate.overrideAttrs(_: {
          postInstall = ''
            cp -R ${../nixos-config/apps/vim-snippets} $out/share/vim-plugins/xptemplate/personal/ftplugin
          '';
        });
      };
    };

    users.extraUsers.vm = {
      isNormalUser = true;
      password = "vm";
    };
  };
}

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.