main.star: annotate missing riscv64 builders with tracking issues

It seems these missing builders need some time before they're back.
Annotate them with tracking issues so that there's less purple on
the gotip post-submit console.

For golang/go#73568.
For golang/go#73569.

Change-Id: Ife7e5c49d01dd9a0c35ae508dd75a0ba7b73f1d6
Reviewed-on: https://go-review.googlesource.com/c/build/+/670595
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: dmitshur