app/appengine: add x/build to the build dashboard

It already does trybots and we try to keep it passing everywhere, so
let's see if that's true.

Fixes golang/go#30640

Change-Id: I9f888efd68eac223f4339736938c74480715a28b
Reviewed-on: https://go-review.googlesource.com/c/build/+/207852
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

app/appengine/dash.go

@@ -106,6 +106,11 @@ var goPackages = []*Package{
 		Name: "blog",
 		Path: "golang.org/x/blog",
 	},
+	{
+		Kind: "subrepo",
+		Name: "build",
+		Path: "golang.org/x/build",
+	},
 	{
 		Kind: "subrepo",
 		Name: "crypto",