freedroidrpg: don't build on hydra

Similar to some other builds, sdl-config gets stuck and times out.

    checking for sdl-config... /nix/store/q2hm1ka0qxs2gv5bmrxj7j80lygk4b5z-SDL-1.2.15-dev/bin/sdl-config
    building of '/nix/store/m2d7v0n5hd2498vfxp18i37p7r1lf76p-freedroidrpg-0.16.1' timed out after 7200 seconds of silence

/cc ZHF #45961

(cherry picked from commit 9b99819)

pkgs/games/freedroidrpg/default.nix

@@ -58,5 +58,6 @@ in stdenv.mkDerivation rec {
 
     maintainers = with maintainers; [ jtojnar ];
     platforms = platforms.unix;
+    hydraPlatforms = platforms.linux; # sdl-config times out on darwin
   };
 }