buildNeovimPlugin: set version accordingly to the manual guidelines (#289008)

https://github.com/NixOS/nixpkgs/blob/master/pkgs/README.md#package-naming

pkgs/applications/editors/neovim/build-neovim-plugin.nix

@@ -33,6 +33,7 @@ in
           nativeBuildInputs = oa.nativeBuildInputs or [] ++ [
             lua.pkgs.luarocksMoveDataFolder
           ];
+          version = "${originalLuaDrv.version}-unstable-${oa.version}";
         }));
     in
       finalDrv