* allow overriding of GITHUB_ env variables
* bug fix for overriding env vars with empty string
* revert step.go
* refactor github_context to prevent lint failures. added more setters
* added ability to override github env variables
* handled base and head ref
* added input flags
* added input as part of the action event and added test cases
* updated readme
Co-authored-by: ChristopherHX <christopher.homberger@web.de>