nixos/munin: enable munin_update and disable munin_stats

munin_update relies on a stats file that exists, but isn't found in the
default location on NixOS; the appropriate plugin configuration is
added.

munin_stats relies on munin-cron writing a logfile, which the NixOS
build of munin does not. (This is probably fixable in the munin package,
but I don't have time to dig into that right now.)