nixos/nixpkgs: pass original system args instead of elaborated

Passing the elaborated system defeats what pkgs/top-level/default.nix
tries to do: Pass only the original args and let defaults be inferred.

The underlying problem is that lib.systems.elaborate can not deal with
arbitrary overrides, but will often return an inconsistent system
description when partially overriding some values. This becomes most
prominent if trying to override an already elaborated system.

lib/modules.nix

@@ -844,6 +844,8 @@ let
 
     in warnDeprecation opt //
       { value = addErrorContext "while evaluating the option `${showOption loc}':" value;
+        # raw value before "apply" above
+        rawValue = addErrorContext "while evaluating the option `${showOption loc}':" res.mergedValue;
         inherit (res.defsFinal') highestPrio;
         definitions = map (def: def.value) res.defsFinal;
         files = map (def: def.file) res.defsFinal;

nixos/modules/misc/nixpkgs.nix

@@ -70,19 +70,24 @@ let
     ++ lib.optional (opt.localSystem.highestPrio < (lib.mkOptionDefault { }).priority) opt.localSystem
     ++ lib.optional (opt.crossSystem.highestPrio < (lib.mkOptionDefault { }).priority) opt.crossSystem;
 
+  # pkgs/top-level/default.nix takes great strides to pass the *original* localSystem/crossSystem args
+  # on to nixpkgsFun to create package sets like pkgsStatic, pkgsMusl. This is to be able to infer default
+  # values again. Since cfg.xxxPlatform and cfg.xxxSystem are elaborated via apply, those can't be passed
+  # directly. Instead we use the rawValue before the apply/elaboration step, via opt.xxx.rawValue.
   defaultPkgs =
     if opt.hostPlatform.isDefined then
       let
+        # This compares elaborated systems on purpose, **not** using rawValue.
         isCross = cfg.buildPlatform != cfg.hostPlatform;
         systemArgs =
           if isCross then
             {
-              localSystem = cfg.buildPlatform;
-              crossSystem = cfg.hostPlatform;
+              localSystem = opt.buildPlatform.rawValue;
+              crossSystem = opt.hostPlatform.rawValue;
             }
           else
             {
-              localSystem = cfg.hostPlatform;
+              localSystem = opt.hostPlatform.rawValue;
             };
       in
       import ../../.. (
@@ -96,9 +101,9 @@ let
         inherit (cfg)
           config
           overlays
-          localSystem
-          crossSystem
           ;
+        localSystem = opt.localSystem.rawValue;
+        crossSystem = opt.crossSystem.rawValue;
       };
 
   finalPkgs = if opt.pkgs.isDefined then cfg.pkgs.appendOverlays cfg.overlays else defaultPkgs;