feat(release): commit changelog and npm files on release branch only

[adekbadek]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Updates the release script so that package.json, package-lock.json, and CHANGELOG.md are not updated when releasing to a channel other than release. Committing these files on alpha release channel generates noise in the changelog (and is not needed there), and creates merge conflicts.

How to test the changes in this Pull Request:

Test according to "Testing locally" instructions in the README

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[chickenn00dle]

Tested and lgtm 👍

[matticbot]

🎉 This PR is included in version 5.4.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀