dashboard: add one more builder for riscv64 unmatched

Thanks to PLCT lab(https://plctlab.github.io)
contributed an unmatched builder.

Change-Id: I68e226edcde9a67de69aaa2599380f14c5dad526
Reviewed-on: https://go-review.googlesource.com/c/build/+/406847
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Zhuo Meng <mzh@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
diff --git a/dashboard/builders.go b/dashboard/builders.go
index ecdf6bd..cd2db12 100644
--- a/dashboard/builders.go
+++ b/dashboard/builders.go
@@ -218,7 +218,7 @@
 	"host-linux-riscv64-unmatched": &HostConfig{
 		Notes:     "SiFive HiFive Unmatched RISC-V board. 16 GB RAM, 4 cores.",
 		IsReverse: true,
-		ExpectNum: 1,
+		ExpectNum: 2,
 		Owners:    []*gophers.Person{gh("mengzhuo")},
 		env:       []string{"GOROOT_BOOTSTRAP=/usr/local/goboot"},
 	},