Add `env_name` to each `FastlaneCore::ConfigItem`

[liamnichols]

Hello @HazAT 👋

We've been using this action for a while and it's been doing a great job! I was recently doing some refactoring however and I want to cleanup some of our fastfiles by doing the following:

- add_badge(shield: "#{title}-#{value}-blue", dark: true, no_badge: true)
+ add_badge(shield: "#{title}-#{value}-blue")

I've done this in a lot of other places by defining our default parameters in a .env file, however I found that we couldn't do it with this action because it doesn't define the env_name on the available options so this PR aims to address that 🙏

It should be a relatively small change all things considered 🙇