Fix the mixin install target

[carolynvs]

What does this change

I was accidentally copying the mixin runtime binary into the wrong
directory under PORTER_HOME.

What issue does it fix

If a developer is using a mixin that was using v1.0.0-* for its magefile install target, then the runtime for the mixin wasn't dropped in the right place for it to be picked up during porter build. This affects: skeletor, docker-compose.

Notes for the reviewer

Oops 🤦‍♀️

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