dashboard: freebsd-riscv64 bootstrap from local

The port of freebsd-riscv64 is still working in progresss
means bootstrap binary from GCP is unavailable for now.

For golang/go#53466

Change-Id: I81ee945e0f0bc95be980e1f2a3d5f61c42e37f28
Reviewed-on: https://go-review.googlesource.com/c/build/+/434295
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

dashboard/builders.go

@@ -241,10 +241,14 @@ var Hosts = map[string]*HostConfig{
 		Owners:    []*gophers.Person{gh("dmgk")},
 	},
 	"host-freebsd-riscv64-unmatched": {
-		IsReverse: true, // afaik
-		ExpectNum: 1,    // actually unspecified
-		Notes:     "SiFive HiFive Unmatched RISC-V board. 16 GB RAM.",
-		Owners:    []*gophers.Person{gh("mengzhuo")},
+		IsReverse:   true,
+		ExpectNum:   1,
+		Notes:       "SiFive HiFive Unmatched RISC-V board. 16 GB RAM. FreeBSD 13.1-RELEASE",
+		Owners:      []*gophers.Person{gh("mengzhuo")},
+		GoBootstrap: "none",
+		env: []string{
+			"GOROOT_BOOTSTRAP=/home/gopher/go-freebsd-riscv64-bootstrap",
+		},
 	},
 	"host-illumos-amd64-jclulow": {
 		Notes:       "SmartOS base64@19.1.0 zone",