Gitlab operator build image: Add 'vcluster' binary

What does this MR do and why?

Add the vcluster binary to the Gitlab Operator build image. This is needed to create v(irtual) Kubernetes clusters during the build and test pipelines of the Gitlab Operator. Using vclusters allows us to more quickly and more reliably test against specific versions of Kubernetes.

• gitlab-org/cloud-native/gitlab-operator#1419 (closed)
• gitlab-org/cloud-native/gitlab-operator!706 (merged)

Checklist

• (If applicable) Add patches if required for upgrading Ruby version in https://gitlab.com/gitlab-org/gitlab-build-images/-/tree/master/patches/ruby.