SimCoreTests ART: Disable for AthSimulation nightlies

Currently Forward Detector simulation is only supported in Athena (not AthSimulation) builds, so this one shouldn't have been enabled for AthSimulation ART runs.
(If Forward Detector simulation is requested more often in the future, then the required packages can be added to the AthSimulation Project.)

NB This MR only affects an ART script, so will have no effect on the CI output.

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