Do not repeat RunActions for worker threads

G4UserRunActions commands should only be constructed for the master thread and not for worker threads. I found in Geant4 examples that in G4UserRunActions are sometimes instantiated for worker threads as well, but in this case, I think that it should be a different set of commands steered from sth like GiGaWorkerRunActionCommand.

cc: @gcorti, @fbilandz

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