dashboard: update linux-riscv64 builder's bootstrap to Go 1.20.7

This is easy to do, and might either resolve the issue or eliminate
the need to consider the Go 1.20.6 bootstrap as contributing to the
problem.

For golang/go#62104.

Change-Id: I2c22c47bb6ef1d84036763492313d1f51fb63fb2
Reviewed-on: https://go-review.googlesource.com/c/build/+/520900
Reviewed-by: Bryan Mills <[email protected]>
Run-TryBot: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>

Author: dmitshur

dashboard/builders.go

@@ -460,6 +460,10 @@ var Hosts = map[string]*HostConfig{
 		IsReverse: true,
 		ExpectNum: 2,
 		Owners:    []*gophers.Person{gh("mengzhuo")},
+
+		// Use go1.20.7 (slightly newer than the default go1.20.6 on 2023-08-18)
+		// for https://go.dev/issue/62104#issuecomment-1683207172.
+		GoBootstrap: "go1.20.7",
 	},
 	"host-linux-s390x": {
 		Notes:     "run by IBM",