nixos/window-managers/i3: write config file to `/etc/i3/config`

The default config of i3 provides a key binding to reload, so changes
take effect immediately:

```
bindsym $mod+Shift+c reload
```

Unfortunately the current module uses the store path of the `configFile`
option. So when I change the config in NixOS, a store path will be
created, but the current i3 process will continue to use the old one,
hence a restart of i3 is required currently.

This change links the config to `/etc/i3/config` and alters the X
startup script accordingly so after each rebuild, the config can be
reloaded.