nixos/xserver: Improve checking keyboard layout

Enumerating the symbols directory doesn't include variants, so we're now
basically doing what "localectl list-x11-keymap-layouts" does but we use
sed instead.

The reason I'm not using localectl directly is because the path to
rules/base.lst is hardcoded in the systemd source.

Of course, the XKB specification allows for much more complicated rules,
but at least this should cover the most basic ones including variants.

So the sed expression itself is just for listing the available layouts
and variants and we use a grep with -xF to match only full lines without
interpreting regular expressions.

This should again allow to set "dvorak" as the layout option.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @lheckemann
Fixes: #25526