Update buildpack.toml to build and publish multi-arch buildpacks

[jericop]

Summary

This change does the following:

• Updates buildpack.toml to build and publish multi-arch buildpacks
• Updates .gitignore with paths for multi-arch builds

Use Cases

multi-arch 🥳

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).