nixos/dconf: fix d-bus activation
nixos/dconf: fix d-bus activation

dconf now supports autostarting the d-bus service using systemd's d-bus activation.

https://gitlab.gnome.org/GNOME/dconf/-/commit/2781a8684855e4120c9948d8b40880bafafe1b12

On NixOS, that requires making systemd aware of the package.

Fixes: https://github.com/NixOS/nixpkgs/issues/123265

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