Fix synths being affected by boiler neurotoxin (#6197)

# About the pull request
For some reason the neurotoxin is an effect instead of a chemical and
lacks the proper checks so it affected synths which is pretty funny
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# Explain why it's good for the game
Consistency and it makes no sense
# Testing Photographs and Procedure
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# Changelog
:cl:
fix: Synths no longer get affected by boiler's neurotoxin
/:cl:

code/datums/effects/neurotoxin.dm

@@ -37,6 +37,10 @@
 		return FALSE
 	if(affected_mob.stat == DEAD)
 		return
+
+	if(issynth(affected_atom))
+		return
+
 // General effects
 	affected_mob.last_damage_data = cause_data
 	affected_mob.apply_stamina_damage(stam_dam)