lib.systems.equals: use lib imports instead of builtins

Use lib.filter and lib.attrNames instead of builtins.* directly,
consistent with the rest of the file's style.

Follow-up to #506588, incorporating review feedback from #507232.

lib/systems/default.nix

@@ -3,11 +3,11 @@
 let
   inherit (lib)
     any
-    filterAttrs
+    attrNames
+    filter
     foldl
     hasInfix
     isAttrs
-    isFunction
     isList
     mapAttrs
     optional
@@ -43,9 +43,8 @@ let
   */
   equals =
     let
-      # perf: avoid lib.isFunction because system attrs are never __functor-style attrsets.
-      removeFunctions =
-        a: removeAttrs a (builtins.filter (n: builtins.isFunction a.${n}) (builtins.attrNames a));
+      # System attrs are never __functor-style attrsets, so builtins.isFunction suffices.
+      removeFunctions = a: removeAttrs a (filter (n: builtins.isFunction a.${n}) (attrNames a));
     in
     a: b: removeFunctions a == removeFunctions b;