distro/nix: sync changes accumulated over longer time

All are quite minor.

distro/pkg/nix/default.nix

 { lib, stdenv, fetchurl, pkg-config, gnutls, liburcu, lmdb, libcap_ng, libidn2, libunistring
 , systemd, nettle, libedit, zlib, libiconv, libintl, libmaxminddb, libbpf, nghttp2, libmnl
-, autoreconfHook, nixosTests
+, autoreconfHook, nixosTests, knot-resolver
 }:
 
-let inherit (lib) optional optionals; in
-
 stdenv.mkDerivation rec {
   pname = "knot-dns";
   version = "{{ version }}";
@@ -36,15 +34,12 @@ stdenv.mkDerivation rec {
     libiconv lmdb libintl
     nghttp2 # DoH support in kdig
     libmaxminddb # optional for geoip module (it's tiny)
-    libmnl # required for knot >= 3.1
     # without sphinx &al. for developer documentation
     # TODO: add dnstap support?
-  ]
-    ++ optionals stdenv.isLinux [
-      libcap_ng systemd
-      libbpf # XDP support
-    ]
-    ++ optional stdenv.isDarwin zlib; # perhaps due to gnutls
+  ] ++ lib.optionals stdenv.isLinux [
+    libcap_ng systemd
+    libbpf libmnl # XDP support (it's Linux kernel API)
+  ] ++ lib.optional stdenv.isDarwin zlib; # perhaps due to gnutls
 
   enableParallelBuilding = true;
 
@@ -58,7 +53,11 @@ stdenv.mkDerivation rec {
     rm -r "$out"/lib/*.la
   '';
 
-  passthru.tests = { inherit (nixosTests) knot; };
+  passthru.tests = {
+    inherit knot-resolver;
+  } // lib.optionalAttrs stdenv.isLinux {
+    inherit (nixosTests) knot;
+  };
 
   meta = with lib; {
     description = "Authoritative-only DNS server from .cz domain registry";