plugins/none-ls: fix withArgs type, syntax (nix-community#1182)

This was previously null or string, but was changed to null or Lua in
nix-community#1169. Internally, `withArgs` is expected to be a
string, but `mkNullOrLua` applies `mkRaw`, leading to a type error when
generating the output, as it is attempting to interpolate a set rather
than a string.

Tested locally, and verified that it fixes my setup.

plugins/none-ls/servers.nix

@@ -331,7 +331,7 @@ in {
         (source: _:
           {
             enable = mkEnableOption "the ${source} ${sourceType} source for none-ls";
-            withArgs = helpers.mkNullOrLua ''
+            withArgs = helpers.mkNullOrOption helpers.nixvimTypes.strLua ''
               Raw Lua code passed as an argument to the source's `with` method.
             '';
           }
@@ -383,7 +383,7 @@ in {
             withArgs =
               if source.withArgs == null
               then sourceItem
-              else "${sourceItem}.with(${source.withArgs}})";
+              else "${sourceItem}.with(${source.withArgs})";
           in
             helpers.mkRaw ''
               require("null-ls").builtins.${withArgs}