treewide: replace pkgs.hostPlatform with pkgs.stdenv.hostPlatform

nixpkgs now throws an error for this, the other variant in stdenv also
exists in the previous release

nixosModules/installer/zfs-latest.nix

@@ -30,5 +30,5 @@ let
 in
 {
   # Note this might jump back and worth as kernel get added or removed.
-  boot.kernelPackages = lib.mkIf (lib.meta.availableOn pkgs.hostPlatform pkgs.zfs) latestKernelPackage;
+  boot.kernelPackages = lib.mkIf (lib.meta.availableOn pkgs.stdenv.hostPlatform pkgs.zfs) latestKernelPackage;
 }