cmd/coordinator: embed UnimplementedCoordinatorServer by value

This type is expected to be embedded by value in order to have the
intended effect. Caught by a newer version of generated code which
panics at startup when an unimplemented server type is embedded as
a pointer.

Change-Id: I5074b8b009a5ff6c86695f727b061dffb9c53065
Reviewed-on: https://go-review.googlesource.com/c/build/+/625055
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

cmd/coordinator/results.go

@@ -25,7 +25,7 @@ import (
 
 type gRPCServer struct {
 	// embed an UnimplementedCoordinatorServer to avoid errors when adding new RPCs to the proto.
-	*protos.UnimplementedCoordinatorServer
+	protos.UnimplementedCoordinatorServer
 
 	// dashboardURL is the base URL of the Dashboard service (https://build.golang.org)
 	dashboardURL string