Fixes AI crew monitor runtime

Instead of the AI crew_monitor being a per-AI variable, it was shared by
all AIs, and recreated each time a new AI was created, as part of that
AI's contents. Bots create and delete a new AI every time they transmit,
which means the crew_monitor would be deleted the moment a bot spoke
over the radio, causing it to runtime.

It is now a per-AI variable.

code/modules/mob/living/silicon/ai/nano.dm

@@ -1,4 +1,5 @@
-var/obj/nano_module/crew_monitor/crew_monitor
+/mob/living/silicon/ai
+  var/obj/nano_module/crew_monitor/crew_monitor
 
 /mob/living/silicon/ai/proc/init_subsystems()
 	crew_monitor = new(src)