Fix interaction between appendOverlays and otherPackageSets

The comment

    a dirty hack that should be removed

has led me to believe that nixpkgsFun isn't the right solution,
but bypassing it is worse, because it creates a second, inner
overriding mechanism that doesn't pass its changes to the old,
outer overriding mechanism.

Before this change:

    nix-repl> ((import <nixpkgs> {}).appendOverlays([(f: s: { foobarbaz = "ok"; })])).foobarbaz
    "ok"

    nix-repl> ((import <nixpkgs> {}).appendOverlays([(f: s: { foobarbaz = "ok"; })])).pkgsCross.aarch64-multiplatform.foobarbaz
    error: attribute 'foobarbaz' missing

           at «string»:1:1:

                1| ((import <nixpkgs> {}).appendOverlays([(f: s: { foobarbaz = "ok"; })])).pkgsCross.aarch64-multiplatform.foobarbaz
                 | ^
                2|

After this change:

    nix-repl> ((import ./. {}).appendOverlays([(f: s: { foobarbaz = "ok"; })])).pkgsCross.aarch64-multiplatform.foobarbaz
    "ok"

Thanks to samueldr for discovering this problem.

1 files changed, 2 insertions, 2 deletions

diff --git a/pkgs/top-level/stage.nix b/pkgs/top-level/stage.nix
index b01ef584d20..84c4d8fc405 100644
--- a/pkgs/top-level/stage.nix
+++ b/pkgs/top-level/stage.nix
@@ -15,7 +15,7 @@
   # Utility functions, could just import but passing in for efficiency
   lib
 
-, # Use to reevaluate Nixpkgs; a dirty hack that should be removed
+, # Use to reevaluate Nixpkgs
   nixpkgsFun
 
   ## Other parameters
@@ -218,7 +218,7 @@ let
     appendOverlays = extraOverlays:
       if extraOverlays == []
       then self
-      else import ./stage.nix (args // { overlays = args.overlays ++ extraOverlays; });
+      else nixpkgsFun { overlays = args.overlays ++ extraOverlays; };
 
     # NOTE: each call to extend causes a full nixpkgs rebuild, adding ~130MB
     #       of allocations. DO NOT USE THIS IN NIXPKGS.