allows mailbags to hold bounty cubes (#92920)

## About The Pull Request
Allows mail bags to hold bounty cubes for optimal manual carrying of
bounty cubes to the cargo shuttle.

## Why It's Good For The Game

if i'm going to run like 10 bounties i don't want to have to carry them
onto the shuttle one by one. alternatively if there's a lot of people
running bounties i'd like to consolidate them into one inventory slot
and not have to lug around a crate to sell it all off

## Changelog

🆑
qol: Mail bags can now hold bounty cubes.
/🆑

Co-authored-by: Hatterhat <Hatterhat@users.noreply.github.com>

code/datums/storage/subtypes/bags.dm

@@ -260,7 +260,8 @@
 	set_holdable(list(
 		/obj/item/mail,
 		/obj/item/delivery/small,
-		/obj/item/paper
+		/obj/item/paper,
+		/obj/item/bounty_cube,
 	))
 
 ///Garment bag