knot-resolver: patch library loading for darwin

Apparently until now it could never start up on x86_64-darwin :-/

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

@@ -39,6 +39,13 @@ unwrapped = stdenv.mkDerivation rec {
     # ExecStart can't be overwritten in overrides.
     # We need that to use wrapped executable and correct config file.
     sed '/^ExecStart=/d' -i systemd/kresd@.service.in
+
+    # On x86_64-darwin loading by soname fails to find the libs, surprisingly.
+    # Even though they should already be loaded and they're in RPATH, too.
+    for f in daemon/lua/{kres,zonefile}.lua; do
+      substituteInPlace "$f" \
+        --replace "ffi.load(" "ffi.load('${lib.getLib knot-dns}/lib/' .. "
+    done
   ''
     # some tests have issues with network sandboxing, apparently
   + optionalString doInstallCheck ''