main.star: add netbsd-arm to SLOW_HOSTS

As mentioned in https://go.dev/issue/63698#issuecomment-2450448780,
this might be a good starting point. Can be adjusted later as needed.

For golang/go#63698.

Change-Id: I851852981527e1dcdae18f535f16a37117791ce6
Reviewed-on: https://go-review.googlesource.com/c/build/+/624075
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Benny Siegert <[email protected]>
Auto-Submit: Benny Siegert <[email protected]>

Author: dmitshur