Execute sTGC and MM Digitization in the same order in old and new configuration (ATLASSIM-5897)

Differing sTGC and MM digitization output was observed between jobs using old and CA-based configuration. This was traced back to the fact that CLHEP::RandGamma leaks information between the sTGC and MM random number streams (see ATLASSIM-5897) and so it makes a difference whether sTGC or MM digitization is run first in a job.
This MR ensures that old and new-style jobs will behave in the same way, but a proper fix for the underlying issue in CLHEP::RandGamma will take more thought.