Unifying cmctl and kubectl cert-manager

[inteon]

Currently, we release two binaries cmctl and kubectl_cert-manager. This was introduced in https://github.com/cert-manager/cert-manager/pull/4522. However, instead of using build-time variables to determine the binary name, we could determine the binary name at runtime (see https://krew.sigs.k8s.io/docs/developer-guide/develop/best-practices/). That would allow us to simplify the releases, we could then only release a single binary.