Replace old style set-output for new GITHUB_ENV

[polsala]

Pull request checklist

Description

Remove deprecation warning from old style set-output

Closes: #103

Types of changes

• This change requires a documentation update
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• Lint and unit tests pass locally with my changes.
• All new and existing tests passed.