Instead of searching `/usr` it should search for the `xkb`,
$XDG_DATA_DIRS will be searched. With this approach we allow compliance
on NixOS and non-NixOS systems to find `symbols` in the `xkb` directory.

The patch has been accepted by upstream, but isn't released yet, so this
is mainly a temporary fix until we can bump ZSH to the next stable version.

The `xserver` module links `/share/X11/xkb` to `/run/current-system` to
make this possible.

The fix can be tested inside the following VM:

```
{
  zshtest = {
    programs.zsh.enable = true;
    users.extraUsers.vm = {
      password = "vm";
      isNormalUser = true;
    };
    services.xserver.enable = true;
  };
}
```

Fixes #46025

zsh: patch `_setxkbmap` completion script