Fixed incorrect arrow on build mode UI. (tgstation#73157)

## About The Pull Request

Resolves tgstation#73110 .

The up-left arrow sprite was a duplicate of the down-left one by
mistake, so I've replaced it with an arrow facing the correct direction.


![image](https://user-images.githubusercontent.com/105025397/216247160-bcc9c9ae-b38b-4c15-a2e2-08780ea447cf.png)
## Why It's Good For The Game

UI buttons should line up with what they actually do.
## Changelog
:cl:
fix: Made the northwest arrow icon in build mode actually point
northwest
/:cl: