generateLuarocksConfig: dont generate rocks_subdir by default

Goal is to make generateLuarocksConfig appropriate in more scenarios, in my case, to generate rocks.nvim luarocks config

We override the luarocks default for rocks_subdir to avoid conflicts when creating lua environments but it is not appropriate for other scenarios
so let's not default to it in the "lib" function.

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

@@ -134,12 +134,15 @@ let
       externalDeps = lib.unique (self.externalDeps ++ externalDepsGenerated);
       local_cache = "";
 
+      # To prevent collisions when creating environments, we install the rock
+      # files into per-package subdirectories
+      rocks_subdir = self.rocksSubdir;
+
       # Filter out the lua derivation itself from the Lua module dependency
       # closure, as it doesn't have a rock tree :)
       # luaLib.hasLuaModule
       requiredLuaRocks = lib.filter luaLib.hasLuaModule
         (lua.pkgs.requiredLuaModules (self.nativeBuildInputs ++ self.propagatedBuildInputs));
-      inherit (self) rocksSubdir;
     };
 
     luarocksConfig' = lib.recursiveUpdate luarocksConfig

pkgs/development/lua-modules/lib.nix

@@ -93,7 +93,6 @@ rec {
       externalDeps ? []
     # a list of lua derivations
     , requiredLuaRocks ? []
-    , rocksSubdir ? "rocks-subdir"
     , ...
     }@args: let
       rocksTrees = lib.imap0
@@ -125,9 +124,6 @@ rec {
 
       generatedConfig = ({
 
-        # To prevent collisions when creating environments, we install the rock
-        # files into per-package subdirectories
-        rocks_subdir = rocksSubdir;
 
         # first tree is the default target where new rocks are installed,
         # any other trees in the list are treated as additional sources of installed rocks for matching dependencies.