Fixes acid pillars shooting captured humans (#5807)

# About the pull request

Fixes #5806 by making
`/obj/effect/alien/resin/acid_pillar/proc/can_target()` check if the
target is nested.

This specifically only checks if they're nested and not if they've
actually been hugged to avoid the same issue happening in the rare cases
where both aren't true.

# Explain why it's good for the game

Acid pillars shouldn't be helping marines by killing captured people.

# Testing Photographs and Procedure
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<summary>Screenshots & Videos</summary>

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screenshots and the `<details>` tags.

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# Changelog
:cl:
fix: Fixed acid pillars shooting nested marines.
/:cl:

code/modules/cm_aliens/XenoStructures.dm

@@ -578,6 +578,9 @@
 	if(current_mob.stat == DEAD)
 		return FALSE
 
+	if(HAS_TRAIT(current_mob, TRAIT_NESTED))
+		return FALSE
+
 	var/turf/current_turf
 	var/turf/last_turf = loc
 	var/atom/temp_atom = new acid_type()