main.star: limit scope of slow netbsd-arm host to go, net, sys

As requested in go.dev/issue/72061, with an adjustment of keeping x/net
and x/sys builders around. Hopefully there's sufficient signal provided
by those to keep them, and it's fine to adjust more if needed.

Fixes golang/go#72061.

Change-Id: Id225c7d6c3d1665ac2a952157e71651c758c32e1
Reviewed-on: https://go-review.googlesource.com/c/build/+/655575
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: dmitshur