nixos/captive-browser: set chromium's data-dir to a XDG-compliant location

To quote the XDG specification:

     There is a single base directory relative to which user-specific
     data files should be written. This directory is defined by the\
     environment variable $XDG_DATA_HOME.

Rather than adding another directory to $HOME, I think that it's better
to follow this standard to avoid a cluttered home-dir.