Fix interaction between appendOverlays and otherPackageSets

[roberth]

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.

The bug was introduced with the introduction of appendOverlays itself: #47430

Motivation for this change

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• 21.11 Release Notes (or backporting 21.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.