Follow-up to #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
