Add output env var to step.yml

[DamienBitrise]

Checklist

• [X] I've read and followed the Contribution Guidelines
• [X] step.yml and README.md is updated with the changes (if needed)

Version

Requires a PATCH version update

Context

To help the customer understand there are env var outputs

Changes

• Add Env Var output

Investigation details

This is a hidden env var created by the step that Fastlane uses (It is used here)[https://github.com/fastlane/fastlane/blob/master/sigh/lib/sigh/options.rb#L65]

Decisions

Helping customers know what is going on