python3Packages.libvirt: Fix build on darwin

The `zfs` package is not supported on darwin at all, resulting in
an evaluation error.
This change conditionally removes the patch that tried to introduce
the dependency regardless of that fact in f00d562080,
https://github.com/NixOS/nixpkgs/pull/191552

pkgs/development/libraries/libvirt/default.nix

@@ -125,6 +125,7 @@ stdenv.mkDerivation rec {
 
   patches = [
     ./0001-meson-patch-in-an-install-prefix-for-building-on-nix.patch
+  ] ++ lib.optionals enableZfs [
     (substituteAll {
       src = ./0002-substitute-zfs-and-zpool-commands.patch;
       zfs = "${zfs}/bin/zfs";