change greetd package to UsrEtc config

[denisok]

Change config location to /usr/etc for greetd in openSUSE.

https://todo.sr.ht/~kennylevinsen/greetd/20

actually we can pass --config as the parameter in service file: https://git.sr.ht/~kennylevinsen/greetd/tree/master/item/greetd.service#L9

to support /usr/etc config instead of /etc/: https://build.opensuse.org/package/show/X11:Wayland/greetd