Using XDG_CONFIG_HOME allows users to place their keyboard configuration into
their home directory and have them loaded automatically.
libxkbcommon now defaults to XDG_CONFIG_HOME/xkb/ first, see
https://github.com/xkbcommon/libxkbcommon/pull/117
However - libxkbcommon uses secure_getenv() to obtain XDG_CONFIG_HOME and thus
fails to load this for the mutter context which has cap_sys_nice.
We need to manually add that search path as lookup path.
As we can only append paths to libxkbcommon's context, we need to start with
an empty search path set, add our custom path, then append the default search
paths.
The net effect is nil where a user doesn't have XDG_CONFIG_HOME/xkb/.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/936
