Correct bst configuration construction for default benchmark test
Description
During the MR to add logging into the tests, a change to the default benchmark test was omitted, and part of the buildstream configuration file requires an extra line to add back in the "sourcedir:" entry to "avoid network fetch time".
Changes proposed in this merge request:
- Add the sourcedir line back in appending to the "usr_config_bst.yml" line which contains the log formatting.
This merge request, when merged, will address issue/bug:
Edited by Lachlan