0AD Installer updated

I’ve taken care of an old issue and updated the 0AD installer to version Alpha 20.
Since this is built against the current SVN trunk, this is actually almost Alpha 21 (a dev told me on IRC it should be released in 1 or 2 weeks).

The download is quite large and there are probably some things we can remove from the archive, patches to the build script are welcome!

Game:
https://lutris.net/games/0-ad/

Script: