Use global sfnt2woff-zopfli in generate script

[lucaspetter]

In generate.py on Linux, try using the global sfnt2woff-zopfli before trying sfnt2woff. This catches cases where compilation would otherwise fail if the user only has sfnt2woff-zopfli installed and not sfnt2woff. (Also, on Ubuntu, the sfnt2woff-zopfli package is available in the repositories, but not sfnt2woff.)