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:

nixosModules/clanCore/vars/flake-module.nix

@@ -12,9 +12,7 @@ in
     { system, pkgs, ... }:
     {
       legacyPackages.evalTests-module-clan-vars = import ./eval-tests {
-        inherit lib;
-        clan-core = self;
-        pkgs = inputs.nixpkgs.legacyPackages.${system};
+        inherit lib pkgs;
       };
       checks.eval-module-clan-vars = pkgs.runCommand "tests" { nativeBuildInputs = [ pkgs.nix-unit ]; } ''
         export HOME="$(realpath .)"
@@ -24,11 +22,15 @@ in
           --show-trace \
           ${inputOverrides} \
           --flake ${
-            self.filter {
-              include = [
-                "flakeModules"
-                "nixosModules"
-                "lib"
+            lib.fileset.toSource {
+              root = ../../..;
+              fileset = lib.fileset.unions [
+                ../../../flake.nix
+                ../../../flake.lock
+                (lib.fileset.fileFilter (file: file.name == "flake-module.nix") ../../..)
+                ../../../flakeModules/clan.nix
+                ../../../lib
+                ../../../nixosModules/clanCore/vars
               ];
             }
           }#legacyPackages.${system}.evalTests-module-clan-vars