Update magefile dependency

[kichristensen]

What does this change

Update magefile dependency to calculate release tag and branch name correctly in Github Actions

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Checklist

• Did you write tests?
• Did you write documentation?
• Did you change porter.yaml or a storage document record? Update the corresponding schema file.
• If this is your first pull request, please add your name to the bottom of our Contributors list. Thank you for making Porter better! 🙇‍♀️