src.ctf.fs: make get_stream_instance_unique_name return `const std::string &`

src.ctf.fs: make get_stream_instance_unique_name return `const std::string &`

We always return the first stream's name.  Return a reference to that
name instead of a new object and a copy of the string.

Change-Id: I657265b74e0566a36f2ae05f6b5b1b6d21f53c66
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8290
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12327
Tested-by: jenkins <jenkins@lttng.org>