Fix arm64 update check issue

[dmikusa]

Summary

This PR adds another jq selector to the query which is running to fetch the old version from buildpack.toml. Previously, this would return two version numbers, one for each version, and that casued a failure in the pipeline. With the additional selector, we get one version, the version for that specific architecture.

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).