Fixes manual revenant event trigger (#79104)

## About The Pull Request

Continuing the work of
https://github.com/tgstation/tgstation/pull/78910, there was another
place that got missed. Observers don't have minds so this will cause a
runtime.

Fixes https://github.com/Skyrat-SS13/Skyrat-tg/issues/24407

## Why It's Good For The Game

Being able to admin spawn revenants again is nice

## Changelog

🆑
fix: admin triggering the Revenant event now works again
/🆑

code/modules/events/ghost_role/revenant_event.dm

@@ -55,7 +55,7 @@
 		return MAP_ERROR
 
 	var/mob/living/basic/revenant/revvie = new(pick(spawn_locs))
-	selected.mind.transfer_to(revvie)
+	revvie.key = selected.key
 	message_admins("[ADMIN_LOOKUPFLW(revvie)] has been made into a revenant by an event.")
 	revvie.log_message("was spawned as a revenant by an event.", LOG_GAME)
 	spawned_mobs += revvie