all: temporarily disable testscript tests

They require a replace directive that breaks "go install". Will revert
this after tagging a new latest release.

cmd/age/age_test.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build ignore
+
 package main
 
 import (