alsa-plugins: don't build with libjack2 support when cross-compiling

libjack2 doesn't cross-compile, see
https://github.com/NixOS/nixpkgs/issues/96548.

The pulseaudio NixOS module uses pkgs.alsaPlugins to create its
asound.conf, so let's make sure that one can be built when
cross-compiling.

pkgs/os-specific/linux/alsa-plugins/default.nix

@@ -15,7 +15,8 @@ stdenv.mkDerivation rec {
   buildInputs =
     [ alsaLib libogg ]
     ++ lib.optional (libpulseaudio != null) libpulseaudio
-    ++ lib.optional (libjack2 != null) libjack2;
+    # https://github.com/NixOS/nixpkgs/issues/96548
+    ++ lib.optional (libjack2 != null && (stdenv.buildPlatform == stdenv.hostPlatform)) libjack2;
 
   meta = with lib; {
     description = "Various plugins for ALSA";