knot-resolver: use embedded lmdb on Darwin for now

I don't know what's wrong there, and Darwin isn't among primary platforms
for kresd.

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

···
 
   nativeBuildInputs = [ pkgconfig which makeWrapper hexdump ];
 
-  buildInputs = [ knot-dns lmdb luajit libuv gnutls ]
+  buildInputs = [ knot-dns luajit libuv gnutls ]
+    ++ optional stdenv.isLinux lmdb # system lmdb causes some problems on Darwin
     ## optional dependencies
     ++ optional doInstallCheck cmocka
     ++ optional stdenv.isLinux systemd # socket activation