main.star: add linux-arm builder

For golang/go#65241.

Change-Id: Ife98e6dc68fb64bd781c04614bad68d200308c4a
Reviewed-on: https://go-review.googlesource.com/c/build/+/558198
TryBot-Bypass: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: dmitshur