babeltrace-log: pass single-trace=yes to sink.ctf.fs

Babeltrace 1's babeltrace-log(1) used to output the trace directly into
the specified directory, not in a subdirectory which is the trace name
(always `STDIN` in this case). Pass single-trace=yes to the sink.ctf.fs
instance to replicate this elaborate strategy.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>