Use default TRT active gas in geo model unless in simulation.

Prevents a warning message: "WARNING accessData: conversion failed for data object 1112780409//TRT/Cond/StatusHT". The TRT GeoModel tries to determine the active gas from the conditions data. This is not yet available when the GeoModel is built outside of simulation. The workaround is to force building the GeoModel with the default gas (Xenon) unless it is a simulation job. The active gas information for digitisation, reconstruction, etc. is read directly from conditions data and not from the GeoModel.

Closes ATLASRECTS-5407