dashboard: add a known issue for linux-riscv64-unmatched
dashboard: add a known issue for linux-riscv64-unmatched

For golang/go#53379.

Change-Id: I758dfac4bf794de151d9c5105954e68224d955b7
Reviewed-on: https://go-review.googlesource.com/c/build/+/412175
Auto-Submit: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

golang/build,golang/build,golang/build,golang/build