nightly: new command to setup nightly builds in CI systems

This lets people using CI systems easily setup a nightly build repo hosted for free in github or gitlab. Right now, its tested on gitlab, but it is quite close to working smoothly on github with Travis CI or CircleCI.

• https://github.com/zom/Zom-Android/pull/355
• fdroidclient!594 (merged)

Here's some example runs:

• https://gitlab.com/eighthave/fdroidclient/-/jobs/41527612
• https://travis-ci.org/zom/Zom-Android/builds/305308461

environment variables:

• https://docs.gitlab.com/ce/ci/variables/README.html
• https://docs.travis-ci.com/user/environment-variables
• https://circleci.com/docs/1.0/environment-variables

There are also some small, semi-related commits included.