Fix BMon not being selected by real event builder in main CBM simulations

Refs #2689

STS was set as reference detector instead of BMon (T0) for event finding, but then Bmon was not being re-added to the list of detectors selected during event building. Due to this it was missing in the output CbmEvent.
The Warning defined at line 1556 of the algo can probably be seen in the log files of the affected runs.

Of interest to @f.kornas

PS: set as draft as not tested locally yet => removed after confirmation of fix by Frederic