Adds covert contractors (VAISO) to the ERT pool (#6623)

# About the pull request

Adds Covert Military Contractors (VAISO) to the ERT pool. Has the same
chance as VAIPO.
(yes it's literally one character worth of change to the code but it's
all I really can do, unless anyone has suggestions)
# Explain why it's good for the game

I think adding a little more variety to the ERTs we currently have will
be good, keeps things different. Along with this, the hostility towards
W-Y can also lead to some interesting RP at times. (something different
from either 'Friendly to the USCM, loot the USCM, or kill everyone)

Also adds the chance of some level of special forces LARP that doesn't
involve being ungodly overpowered.

# Testing Photographs and Procedure
Opened the game on my machine, called a few randomized distress beacons
until I got the covert contractors.
<details>
<summary>Screenshots & Videos</summary>


![image](https://github.com/cmss13-devs/cmss13/assets/140837937/fb959ac1-4f77-4f8d-b471-ab79079eec42)


</details>


# Changelog

:cl:
add: Covert contractors added to the ERT pool
/:cl:

code/datums/emergency_calls/contractor.dm

@@ -87,7 +87,7 @@
 /datum/emergency_call/contractors/covert
 	name = "Military Contractors (Covert) (Hostile to WY)"
 	mob_max = 7
-	probability = 0
+	probability = 20
 	max_medics = 1
 	max_engineers = 1
 	max_heavies = 1