[git_branch] Adding AppCenter for ENV variable check

[matemoln]

🔑

Checklist

• I've run bundle exec rspec from the root directory to see all new and existing tests pass
• I've followed the fastlane code style and run bundle exec rubocop -a to ensure the code style is valid
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.

Motivation and Context

When running fastlane on AppCenter git_branch action returns HEAD. This have been fixed for gitlab-ci and other CI providers but not for AppCenter.

Description

Added APPCENTER_BRANCH environment variable to the list of supported git branch environment variables.

[googlebot]

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[matemoln]

@googlebot I signed it!

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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[googlebot]

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[fastlane-bot]

Hey @matemoln 👋

Thank you for your contribution to fastlane and congrats on getting this pull request merged 🎉
The code change now lives in the master branch, however it wasn't released to RubyGems yet.
We usually ship about once a week, and your PR will be included in the next one.

Please let us know if this change requires an immediate release by adding a comment here 👍
We'll notify you once we shipped a new release with your changes 🚀

[fastlane-bot]

Congratulations! 🎉 This was released as part of fastlane 2.147.0 🚀