Only run yarn:install before assets:precompile if bin/yarn present

Based on discussion in rails#40795, it
looks like `yarn:install` is *always* run, even if the Rails project
disabled javascript and there is no `bin/yarn`.

Check for the existence of `bin/yarn` to decide if `yarn:install` should
be run or not.

The check for this is taken from `railties/lib/rails/app_updater.rb`,
where it does the same:

```ruby
         options[:skip_javascript] = !File.exist?(Rails.root.join("bin", "yarn"))
```

railties/lib/rails/tasks/yarn.rake

@@ -25,6 +25,6 @@ namespace :yarn do
 end
 
 # Run Yarn prior to Sprockets assets precompilation, so dependencies are available for use.
-if Rake::Task.task_defined?("assets:precompile")
+if Rake::Task.task_defined?("assets:precompile") && File.exist?(Rails.root.join("bin", "yarn"))
   Rake::Task["assets:precompile"].enhance [ "yarn:install" ]
 end