lastest-zfs-kernel: fix eval on unsupported platforms

riscv64 for example.

nixos/mixins/latest-zfs-kernel.nix

@@ -24,5 +24,5 @@ let
 in
 {
   # Note this might jump back and worth as kernel get added or removed.
-  boot.kernelPackages = latestKernelPackage;
+  boot.kernelPackages = lib.mkIf (lib.meta.availableOn pkgs.hostPlatform pkgs.zfs) latestKernelPackage;
 }