fix(system-upgrade): fix still broken scraping config

This patch should finally resolve the problem for the scraping of the
metrics-server. It seems `kubeadm-flux.conf` was a bad name for the
override as it was loaded *before* the regular `kubeadm.conf` override.

Therefore this patch changes the name to `zz_flux.conf`, which should
**definitely** be loaded afterwards and result in the expected CLI
parameters being used.