checks/eval-module-clan-vars: optimize to use filtered source

- Replace self.filter with lib.fileset for more precise filtering
- Remove unnecessary clan-core dependency from the test
- Test only needs lib and pkgs, not the full flake context
- Prevents unnecessary rebuilds when unrelated files change

nixosModules/clanCore/vars/eval-tests/default.nix

@@ -1,4 +1,4 @@
-{ lib, pkgs, ... }:
+{ lib, pkgs }:
 let
   eval =
     module: