example/ivy: use tools build constraint in tools.go

In modern versions of the go command, the go mod tidy command doesn't
consider module requirements coming from files with an "ignore" build
constraint. Use "tools" instead.

This way, running go mod tidy on this module becomes a no-op, instead
of deleting all of the requirements from go.mod and go.sum.

Change-Id: I42a24c462acf7340410614fa3b0dd80608efcdaa
Reviewed-on: https://go-review.googlesource.com/c/mobile/+/626655
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

example/ivy/tools.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build ignore
+//go:build tools
 
 package dummy