nixpkgs: fix installation on non-x86

On non-x86 architectures (for example, aarch64) the installation of
home-manager fails indicating that it is attempting to select i686
packages for Linux and those aren't available.

Solution: make the condition for choosing these packages stricter

modules/misc/nixpkgs.nix

@@ -144,7 +144,10 @@ in
   config = {
     _module.args = {
       pkgs = _pkgs;
-      pkgs_i686 = if _pkgs.stdenv.isLinux then _pkgs.pkgsi686Linux else {};
+      pkgs_i686 =
+        if _pkgs.stdenv.isLinux && _pkgs.stdenv.hostPlatform.isx86
+        then _pkgs.pkgsi686Linux
+        else { };
     };
   };
 }