main.star: add windows-arm64 builders

For golang/go#64587.

Change-Id: Ic71ff512f2f099017991fd856d7b2fa5aefea5f0
Reviewed-on: https://go-review.googlesource.com/c/build/+/547937
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Than McIntosh <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Commit-Queue: Than McIntosh <[email protected]>
TryBot-Bypass: Dmitri Shuralyov <[email protected]>

Author: thanm