Fixes doubling of gadget points for second+ synth (#3936)

# About the pull request

Fixes a bug where if any synthetic that joins will get 45 extra gadget
points more than the previous, so 2nd synthetic gets 90 points, 3rd
would have 135, and so on. Tested in a private server and working
properly.

# Explain why it's good for the game

Makes it so late join synthetics or additional joins from events don't
get massive gadget budget to spend on like 3 compact defibrillators or
start taking things to hand out to marines.

# Testing Photographs and Procedure

Went into private server running the branch, spawned 3 shipside USCM
synthetics. Sequentially vended the coin, then took 3 gadgets, then did
the same with the next. Pre-patch this would result in the 2nd and 3rd
synthetics getting huge point budgets, with this patch correctly only
gave 45 points to each synthetic.

</details>


# Changelog

:cl:
fix: Fixed synthetics getting an extra 45 points more than any previous
who had used the gadget vendor
/:cl:

code/game/machinery/vending/vendor_types/crew/synthetic.dm

@@ -295,7 +295,7 @@ GLOBAL_LIST_INIT(cm_vending_clothing_synth_snowflake, list(
 /obj/structure/machinery/cm_vending/own_points/experimental_tools/attackby(obj/item/W, mob/user)
 	if(istype(W, /obj/item/coin/marine/synth))
 		if(user.drop_inv_item_to_loc(W, src))
-			available_points = available_points + 45
+			available_points = 45
 			available_points_to_display = available_points
 			to_chat(user, SPAN_NOTICE("You insert \the [W] into \the [src]."))
 			return