Prevent Artifact Stones from duplicating when picked up the same tic it respawns.
A rather simple fix.
Originally when you picked up an Artifact stone (Possession/Terminator) on the same frame it that respawns, it would result in the stone being duplicated.
This MR adds a GAMEMODE_GetArtifactCarrier check inside of GAME_SelectRandomSpotForArtifact and prevents spawning if there's an artifact carrier.