Fix undefined package_json during Yarn install (#676)

When running `rails stimulus_reflex:install` on a new Rails app using
Yarn & esbuild, the following error is raised:

```
  --- [yarn] ----
  rails aborted!
  NameError: undefined local variable or method `package_json'
  for #<Rails::Generators::AppGenerator...
```

Looks like it's a missing update from
#655

## Why should this be added
Setup raises errors at the Yarn step

lib/install/yarn.rb

@@ -13,7 +13,7 @@
 dev = dev_package_list.exist? ? dev_package_list.readlines.map(&:chomp) : []
 drop = drop_package_list.exist? ? drop_package_list.readlines.map(&:chomp) : []
 
-json = JSON.parse(package_json.read)
+json = JSON.parse(package_json_path.read)
 
 if add.present? || dev.present? || drop.present?