nixos/x11: set XKB_CONFIG_ROOT from config

The motivation is to make overriding `services.xserver.xkbDir` easier.

Changed files
+1 -1
nixos
modules
services
+1 -1
nixos/modules/services/x11/extra-layouts.nix
···
environment.sessionVariables = {
# runtime override supported by multiple libraries e. g. libxkbcommon
# https://xkbcommon.org/doc/current/group__include-path.html
-
XKB_CONFIG_ROOT = "${xkb_patched}/etc/X11/xkb";
+
XKB_CONFIG_ROOT = config.services.xserver.xkbDir;
};
services.xserver = {