main.star: remove freebsd-riscv64 known issue

The builder is up and working. There are occasional temporary network
errors, but that can be improved upon separately.

Fixes golang/go#63482.

Change-Id: I5ebaf61aee4c8f3ec0031c6b9544126d5343ec20
Reviewed-on: https://go-review.googlesource.com/c/build/+/586036
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>

Author: mengzhuo