tests.bittorrent: use a file instead of a directory

nixUnstable.src is a directory, which made cp fail without -r

Changed files
+1 -1
nixos
+1 -1
nixos/tests/bittorrent.nix
···
let
# Some random file to serve.
-
file = pkgs.nixUnstable.src;
+
file = pkgs.hello.src;
miniupnpdConf = nodes: pkgs.writeText "miniupnpd.conf"
''