main.star: unset known issues for plan9/{386,arm} builder types

These builders have reached a point where they're considered added.

Fixes golang/go#63599.
Fixes golang/go#63601.

Change-Id: I37bcd2905b0b39b639828c3e3de9f2d0ebdbc994
Reviewed-on: https://go-review.googlesource.com/c/build/+/608155
Reviewed-by: Michael Pratt <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: David du Colombier <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Richard Miller <[email protected]>

Author: dmitshur