knot-resolver: fix a stupid system breaking non-systemd

I hope this will fix darwin build (no good way of testing locally).

pkgs/servers/dns/knot-resolver/default.nix

@@ -74,6 +74,7 @@ unwrapped = stdenv.mkDerivation rec {
   postInstall = ''
     rm "$out"/lib/libkres.a
     rm "$out"/lib/knot-resolver/upgrade-4-to-5.lua # not meaningful on NixOS
+  '' + optionalString stdenv.targetPlatform.isLinux ''
     rm -r "$out"/lib/sysusers.d/ # ATM more likely to harm than help
   '';