test/helpers: use rc.0 as the default version of kubectl
test/helpers: use rc.0 as the default version of kubectl

Since we only update the Kubernetes version tested on our CI when the
first RC is announced we should use that binary instead of the `.0` as
the `.0` is not available at the time the rc.0 is released.

Fixes: 61812551f659 ("test: ensure kubectl version is available for test run")
Signed-off-by: André Martins <bc9800b9d52a24cce72a73dd528afed53f10e5fc@cilium.io>

cilium/cilium,michi-covalent/cilium,cilium/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,cilium/cilium,michi-covalent/cilium,michi-covalent/cilium