main.star: add per-port builder list views

This is useful for seeing how a port is doing at a glance.

While we're here, also rename the prod-builders list view to be more
consistent with the rest of https://ci.chromium.org/p/golang.

Change-Id: I77ba3568865ce92d623a0dbaa05134abbfdaa47e
Reviewed-on: https://go-review.googlesource.com/c/build/+/534219
Reviewed-by: Heschi Kreinick <[email protected]>
TryBot-Bypass: Michael Knyszek <[email protected]>

Author: mknyszek