PyJobTransforms: use default asetup in sub-steps running in different release

When running a sub-step (e.g. RDOtoRDOTrigger) in a different release,
use the `asetup` alias (defined by `setupATLAS`) to setup the release
instead of the `$AtlasSetup` from the user environment. The latter may
point to the beta version on afs that is not accessible from within the
container.