Comment the unknown `BRANCH_NAME` env var; we shouldn't use it

[jpwilliams]

Summary

Support for the BRANCH_NAME environment variable was added in #158, but has no note of why we support it.

Checklist

• [ ] ~Added a docs PR that references this PR~ N/A
• [ ] ~Added unit/integration tests~ N/A
• [x] Added changesets if applicable

Related

• 158

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Latest commit: e7592b164e9f3991e8b3230966d460aad5569c4d

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