generateLuarocksConfig: dont hardcode local_cache

I tried using luaLib.generateLuarocksConfig outside nixpkgs, to generate a rocks.nvim config but it adds a `local_cache = ""` configuration line that is painful to remove so let's not add it in the first place, and let the caller set it if needed.

pkgs/development/interpreters/lua-5/build-luarocks-package.nix

@@ -132,6 +132,8 @@ let
 
     generatedConfig = luaLib.generateLuarocksConfig {
       externalDeps = lib.unique (self.externalDeps ++ externalDepsGenerated);
+      local_cache = "";
+
       # Filter out the lua derivation itself from the Lua module dependency
       # closure, as it doesn't have a rock tree :)
       # luaLib.hasLuaModule

pkgs/development/lua-modules/lib.nix

@@ -124,7 +124,6 @@ rec {
         (lib.filter (lib.isDerivation) externalDeps);
 
       generatedConfig = ({
-        local_cache = "";
 
         # To prevent collisions when creating environments, we install the rock
         # files into per-package subdirectories