(GitHub) Default env vars naming

[mateusz-was]

Is there an existing issue for this?

• I have searched the existing issues

Description of the bug

Hi!

This is more 'quality-of-life' enhancement rather than bug, bug I guess it might be helpful...

There are 2 sets naming standards for default GitHub env var values:

1. GitHub CLI values (prefixed GH_): https://cli.github.com/manual/gh_help_environment
2. Default env vars set by GitHub in Actions (prefixed GITHUB_): https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/store-information-in-variables#default-environment-variables

For default repository value git-cliff uses GITHUB_REPO which doesn't fall inline with any of the above.
GH_REPO is used in 1) and GITHUB_REPOSITORY is used in 2)

This can be confusing when you use git-cliff in GitHub Actions for the first time (directly from CLI, without https://github.com/orhun/git-cliff-action).

Steps To Reproduce

1. Setup GitHub Action pipeline

jobs:
  generate-changelog:
    name: Generate changelog
    runs-on: linux
    steps:
    - name: Checkout
      uses: actions/checkout@v3
      with:
        fetch-depth: 0
        token: '${{ secrets.GIT_CLIFF_PAT }}'
    - name: Install git-cliff
      run: pip install git-cliff
    - name: Render changelog
      run: |
        env | grep GITHUB
        git cliff -v --github-repo=$GITHUB_REPOSITORY --github-token=${{ secrets.GIT_CLIFF_PAT }}` 

Expected behavior

--github-repo=$GITHUB_REPOSITORY should be obsolete and it's value taken from existing GITHUB_REPOSITORY default env variable for better user experience

Screenshots / Logs

No response

Software information

• Operating system: GitHub Actions
• Rust version: - (used from PyPI)
• Project version: 2.7.0

Additional context

This hurt me more, because there was GITHUB_REPO variable set in my company's GHE server. It was pointing to some GitHub Actions configuration repository. I spend an hour to find why the API call were failing 🤷‍♂️

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