pkgs/top-level: fix composing "native cross" package sets

When using pkgsCross with a system that ends up the same as the
localSystem, then modifications for package sets like pksgMusl need to
be done for **both** localSystem and crossSystem. Consider the following
on x86_64-linux:

  pkgsCross.gnu64.pkgsMusl

Before this change, this would result in a musl buildPlatform, but a gnu
hostPlatform. This breaks the promise of "stacking" package sets on top
of each other.

After this change, it results in a musl buildPlatform and a musl
hostPlatform. This works better.

One could expect this to result in the same as pkgsCross.musl64, i.e. a
gnu buildPlatform and a musl hostPlatform, however I couldn't get this
to work without increasing memory usage for ci/eval by many, many GB.
This is caused by usage of pkgsi686Linux inside the main package set,
which follows the same hybrid pattern.

pkgs/test/top-level/stage.nix

@@ -86,9 +86,8 @@ assert isComposable "pkgsStatic";
 assert isComposable "pkgsi686Linux";
 
 # Special cases regarding buildPlatform vs hostPlatform
-# TODO: fails
-# assert discardEvaluationErrors (pkgsCross.gnu64.pkgsMusl.stdenv.hostPlatform.isMusl);
-# assert discardEvaluationErrors (pkgsCross.gnu64.pkgsi686Linux.stdenv.hostPlatform.isx86_32);
+assert discardEvaluationErrors (pkgsCross.gnu64.pkgsMusl.stdenv.hostPlatform.isMusl);
+assert discardEvaluationErrors (pkgsCross.gnu64.pkgsi686Linux.stdenv.hostPlatform.isx86_32);
 assert discardEvaluationErrors (pkgsCross.mingwW64.pkgsLinux.stdenv.hostPlatform.isLinux);
 assert discardEvaluationErrors (
   pkgsCross.aarch64-darwin.pkgsx86_64Darwin.stdenv.hostPlatform.isx86_64

pkgs/top-level/stage.nix

@@ -191,10 +191,13 @@ let
         (lib.systems.systemToAttrs (lib.defaultTo prevArgs.localSystem prevArgs.crossSystem or null)) // crossAttrs;
     });
     # This is only cross when we are already cross, otherwise local.
-    mkHybridPkgs = name: hybridAttrs: mkPkgs name (prevArgs: {
-      ${if stdenv.hostPlatform == stdenv.buildPlatform then "localSystem" else "crossSystem"} =
-        (lib.systems.systemToAttrs (lib.defaultTo prevArgs.localSystem prevArgs.crossSystem or null)) // hybridAttrs;
-    });
+    # For the case of "native cross", i.e. pkgsCross.gnu64 on a x86_64-linux system, we need to adjust **both**
+    # localSystem **and** crossSystem, otherwise they're out of sync.
+    mkHybridPkgs = name: hybridAttrs: mkPkgs name (prevArgs: let
+        newSystem = (lib.systems.systemToAttrs (lib.defaultTo prevArgs.localSystem prevArgs.crossSystem or null)) // hybridAttrs;
+      in lib.optionalAttrs (stdenv.hostPlatform == stdenv.buildPlatform) { localSystem = newSystem; }
+      // lib.optionalAttrs (prevArgs ? crossSystem) { crossSystem = newSystem; }
+    );
   in self: super: {
     # This maps each entry in lib.systems.examples to its own package
     # set. Each of these will contain all packages cross compiled for