dashboard: default unmatched as riscv64 slowbot
dashboard: default unmatched as riscv64 slowbot

Unmatched builders are faster than unleashed now.
We can set unmatched as default slowbot for riscv64.

Change-Id: Ie4ad4575a5b8364b7cdb6a6dab99ce7f61556bf1
Reviewed-on: https://go-review.googlesource.com/c/build/+/407794
Run-TryBot: Zhuo Meng <3e6d300a356a3b86bc195677414a9ec5508ca893@golangcn.org>
Reviewed-by: Joel Sing <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@sing.id.au>
Reviewed-by: Bryan Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>

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