Fix GetArmyBrain being called on observers in sim side of recall

[lL1l1]

Description of the proposed changes

Caused by #6396 with 81a2588.
Fixes the following error:

warning: Error running lua script: Invalid army -1. (Use a 1-based index)
         stack traceback:
         	[C]: in function `GetArmyBrain'
         	...\fa\lua\sim\recall.lua(114): in function `ArmyRecallRequestCooldown'
         	...\fa\lua\sim\recall.lua(474): in function <...\fa\lua\sim\recall.lua:465>

Testing done on the proposed changes

Switching to observer no longer generates the error.

Additional context

Checklist

- [ ] Changes are annotated, including comments where useful

• Changes are documented in the changelog for the next game version