DEV: Don't run dev-only code in specs (#23427)

This was causing the following notice to be printed out when running system specs:

```
I did no detect a custom `config/dev.yml` file, creating one for you where you can amend defaults.
```

(since 61571bee43eae88755b5e258e96d03c146f9d2cb)

lib/faker/discourse_markdown.rb

@@ -86,7 +86,7 @@ module Faker
 
       def available_methods
         methods = super
-        methods << :image if ::DiscourseDev.config.post[:include_images]
+        methods << :image if Rails.env.dev? && ::DiscourseDev.config.post[:include_images]
         methods
       end
     end