[Bandwidth tests] Use LBN_BUILD_JOBS to properly set n_threads on Moore in BW test

FYI @clemenci

The Moore bandwidth tests were recently moved over to a new 64-core machine, but they should only be using 16 threads for the Moore event loop. They're currently using everything through nproc - this changes it to use the LBN_BUILD_JOBS environment variable, defaulting to single-thread if this variable not set (e.g. when running locally).