nixos/ldso: avoid instance of nixpkgs (NixOS#288509)

Follow-up to NixOS#269551

Avoid creating a new instance of nixpkgs to access two variables.
`pkgs.pkgsi686Linux` was being accessed whenever the feature is being
used or not.

A second instance of nixpkgs is being created in
`nixos/modules/config/stub-ld.nix` and can be disabled by setting
`environment.ldso32 = null` or `environment.stub-ld.enable = false`.

Both combined fixes this error:

    error: attribute 'i686-linux' missing

nixos/modules/config/ldso.nix

@@ -6,9 +6,9 @@ let
   libDir = pkgs.stdenv.hostPlatform.libDir;
   ldsoBasename = builtins.unsafeDiscardStringContext (last (splitString "/" pkgs.stdenv.cc.bintools.dynamicLinker));
 
-  pkgs32 = pkgs.pkgsi686Linux;
-  libDir32 = pkgs32.stdenv.hostPlatform.libDir;
-  ldsoBasename32 = builtins.unsafeDiscardStringContext (last (splitString "/" pkgs32.stdenv.cc.bintools.dynamicLinker));
+  # Hard-code to avoid creating another instance of nixpkgs. Also avoids eval errors in some cases.
+  libDir32 = "lib"; # pkgs.pkgsi686Linux.stdenv.hostPlatform.libDir
+  ldsoBasename32 = "ld-linux.so.2"; # last (splitString "/" pkgs.pkgsi686Linux.stdenv.cc.bintools.dynamicLinker)
 in {
   options = {
     environment.ldso = mkOption {