Create repositories for the nightly builds

[fangfufu]

I think it would be great if we create binary repositories for tinc, since we are already creating nightly builds. This way it is easier to keep tinc updated.

For Debian in particular, it seems to be possible to host the repository on Github itself: https://pmateusz.github.io/linux/2017/06/30/linux-secure-apt-repository.html https://github.com/rpatterson/github-apt-repos https://assafmo.github.io/2019/05/02/ppa-repo-hosted-on-github.html

[splitice]

Stable builds would be really useful too