Switch off QuietMode by default

Since we reduced the verbosity of simulation we've been seeing some crashes in simulation ART jobs which seem to indicate that a race-condition might have been uncovered (see ATLASSIM-7273 for details). (HITS files are unaffected, but the job sometimes returns 129 rather than 0.) This merge request (hopefully temporarily) returns the output to its previous verbosity by default, but the reduced verbosity can still be tested by adding flags.Exec.QuietMode=Truein a preExec.