MB-8029 Pass tmp dir to view file sorter command

To reflect the latest changes in couch_view_file_sorter
(couchstore), pass the full path of the directory used
to store temporary index files, so that the file sorter
module uses that directory for its temporary files.

Change-Id: I724cf2511427e094193654deed8d948c97f63dc8
Reviewed-on: http://review.couchbase.org/26193
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>