nixos/munin: change activationScript to preStart script

All activation scripts run in serial upon boot and nixos-rebuild switch
etc., in contrast to preStart which run before a service starts, and can
run in parallel with other services.

The munin(-node) activation script is particularly slow. Change it to a
preStart script so that it can run in parallel with other services and
not slow down boot (or nixos-rebuild switch).

This reduces (repeated) "nixos-rebuild test" time from ~16 seconds to ~8
on my (old) laptop.