top-level: Simplify impure and pure fallback

This is now possible, since the `platform` attribute has been removed in
PR #107214. I've been waiting to do a cleanup like this for a long time!

pkgs/top-level/default.nix

@@ -1,9 +1,8 @@
 /* This function composes the Nix Packages collection. It:
 
-     1. Applies the final stage to the given `config` if it is a function
+     1. Elaborates `localSystem` and `crossSystem` with defaults as needed.
 
-     2. Infers an appropriate `platform` based on the `system` if none is
-        provided
+     2. Applies the final stage to the given `config` if it is a function
 
      3. Defaults to no non-standard config and no cross-compilation target
 
@@ -50,6 +49,14 @@ let # Rename the function arguments
 in let
   lib = import ../../lib;
 
+  localSystem = lib.systems.elaborate args.localSystem;
+
+  # Condition preserves sharing which in turn affects equality.
+  crossSystem =
+    if crossSystem0 == null || crossSystem0 == args.localSystem
+    then localSystem
+    else lib.systems.elaborate crossSystem0;
+
   # Allow both:
   # { /* the config */ } and
   # { pkgs, ... } : { /* the config */ }
@@ -58,17 +65,6 @@ in let
     then config0 { inherit pkgs; }
     else config0;
 
-  # From a minimum of `system` or `config` (actually a target triple, *not*
-  # nixpkgs configuration), infer the other one and platform as needed.
-  localSystem = lib.systems.elaborate (if builtins.isAttrs args.localSystem then (
-    # Allow setting the platform in the config file. This take precedence over
-    # the inferred platform, but not over an explicitly passed-in one.
-    builtins.intersectAttrs { platform = null; } config1
-    // args.localSystem) else args.localSystem);
-
-  crossSystem = if crossSystem0 == null then localSystem
-                else lib.systems.elaborate crossSystem0;
-
   configEval = lib.evalModules {
     modules = [
       ./config.nix