kubectl-mongodb promotion: Use short=8 format

[MaciejKaras]

Summary

Issue found by @fealebenpae during latest release:

I’m seeing a weird problem - build_kubectl_mongodb_plugin from the PR merge commit build on master publishes the kubectl plugin to s3://mongodb-kubernetes-staging/kubectl-mongodb/b3bb7bbb/kubectl-mongodb_darwin_amd64, but the release_kubectl_mongodb_plugin task in the git tag build is looking for it in s3://mongodb-kubernetes-staging/kubectl-mongodb/b3bb7bbba/kubectl-mongodb_darwin_amd64 and I can’t figure out why the version string is shorter like that in one case

Proof of Work

N/A

Checklist

• Have you linked a jira ticket and/or is the ticket in the title?
• Have you checked whether your jira ticket required DOCSP changes?
• Have you added changelog file?
  • use skip-changelog label if not needed
  • refer to Changelog files and Release Notes section in CONTRIBUTING.md for more details

[github-actions]

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MCK 1.6.1 Release Notes