Signed pre-releases into a kolibri-proposed PPA via Github Actions

[benjaoming]

At some stage, we should start experimenting with allowing Buildkite to push signed source packages into a proposed PPA for testers an devs.

[benjaoming]

I have carefully written down the various steps in our README now, to sort of compensate for the fact that this is probably not going to happen....

On a high-level, I have been stalling this a lot, because basically every time I build the Debian sources, I discover new things. So building them hasn't yet become a tedious routine, but more like a part of development.

For the following reasons, it seems too difficult:

1. Source packages have their own change logs. These are manually maintained, and having packages placed in the PPA without any proper manual intervention seems wrong for this reason. We'll have untested packages in the PPA, which cannot be overwritten and haven't been tested.

2. The Debian version isn't the same for pre-releases, have to add a ~ in pre-releases

3. After uploading the source, the PPA will build the base trusty version. This likely takes 5-10 minutes before it's built and published.

4. Copying packages in the PPA requires a package script (see changes in Add python3-distutils to requirements + readme updates #35) - and this requires login via Oauth. I don't know if this can be scripted, I did it with a browser. Copying the packages seems to be a terrible idea for Buildkite because it would require it to halt for 10 minutes to wait for Launchpad to build.

5. I would vouch that the REAL source for building packages is in the PPA, obtained via apt-get source kolibri, but it can also be argued that in some cases, we would want Buildkite to be combining some branch in the Git kolibri-installer-debian repo with the latest source package.

Another way to put it: Buildkite is a cloud-based service for building stuff... so is Launchpad. We already leverage Launchpad for what it was built for.

[benjaoming]

CC @DXCanas