main.star: turn up windows-386

This completes the non-misc-compile trybot set.

Change-Id: Ie536ef322f2a80f361bdbae813e0e7599f1aa7ab
Reviewed-on: https://go-review.googlesource.com/c/build/+/514015
Auto-Submit: Heschi Kreinick <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Bypass: Dmitri Shuralyov <[email protected]>

Author: heschi