Add vendor directory back to the repo

[hisaac]

Checklist

• [x] I've read and followed the Contribution Guidelines
• [x] step.yml and README.md is updated with the changes (if needed)

Version

Requires a MAJOR/MINOR/PATCH version update

Context

I removed the vendor directory in my last PR (https://github.com/bitrise-steplib/steps-xcode-build-for-simulator/pull/32), not realizing that we have a standard of including the directory for our step repos.

Changes

• Add the vendor directory back to the repo.