plugins/treesitter: remove vim.cmd hack

modules/plugins/treesitter/config.nix

@@ -37,11 +37,10 @@ in {
       };
 
       # For some reason treesitter highlighting does not work on start if this is set before syntax on
-      # HACK: is there a way to convert the foldexpr line to lua?
       luaConfigRC.treesitter-fold = mkIf cfg.fold (entryBefore ["basic"] ''
         -- This is required by treesitter-context to handle folds
         vim.o.foldmethod = "expr"
-        vim.cmd("set foldexpr=nvim_treesitter#foldexpr())"
+        vim.o.foldexpr = "nvim_treesitter#foldexpr()"
 
         -- This is optional, but is set rather as a sane default.
         -- If unset, opened files will be folded by automatically as