github CI: nightly builds for multiple distributions

[neolynx]

Fixes #1251

Description of the Change

Since the pipeline changed to use ubuntu22.04 runners, the nighty debian packages do not work on debian buster anymore.
This change updates the pipeline to build for Ubuntu 20.04 and 22.04 as well as for
Debian 10, 11 and 12.

The distribution specific apt sources are as follows:

  "deb http://repo.aptly.info/nightly-bookworm bookworm main"

(replace bookworm with focal, buster, bullseye. Install aptly repo key with: curl -fsS https://www.aptly.info/pubkey.txt | sudo gpg --dearmor -o /etc/apt/trusted.gpg.d/aptly.gpg)

The builds on focal will also release to the legacy nightly apt repo: https://github.com/aptly-dev/aptly/actions/runs/8723898496/job/23933824692#step:7:24

This only affects nightly builds, for now.
Pipeline example: [](https://github.com/aptly-dev/aptly/actions/runs/8723898496)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 74.70%. Comparing base (3a29e08) to head (da9933d). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1270 +/- ## =========================================== + Coverage 0 74.70% +74.70% =========================================== Files 0 144 +144 Lines 0 16216 +16216 =========================================== + Hits 0 12114 +12114 - Misses 0 3158 +3158 - Partials 0 944 +944 ```

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