Bump magefiles to v0.3.2

[carolynvs]

What does this change

Update to the most recent version of magefiles, mage and magex. I'm doing this manually since dependabot doesn't know to bump them all together.

What issue does it fix

N/A

Notes for the reviewer

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! 🙇‍♀️

Reviewer Checklist

• Comment with /azp run test-porter-release if a magefile or build script was modified
• Comment with /azp run porter-integration if it's a non-trivial PR