ci: build .deb packages for Debian/Ubuntu and publish them as an apt repository

[denyspozniak]

Two things: build .deb for Ubuntu as well as Debian, and serve them from a repository
hosted by this repo — an interim solution until there's a properly signed archive.

Closes #559

• /debs in each image, one artifact per suite, flat apt repo per suite, deployed to Pages
• separate directory per suite — builds aren't interchangeable across glibc versions
• adds Ubuntu 22.04 and 24.04, which had no image; both reuse existing packaging
  (jammy → bullseye, noble → bookworm), no new pkg/deb/
• versions from run_number (2.1.0+ci42~debian12) — apt compares version strings only,
  so a rebuilt 2.1.0 never reaches clients; ordering checked with
  dpkg --compare-versions
• the five deb images had drifted apart in style; unified, dependency sets unchanged
• needs Settings → Pages → Source: GitHub Actions; publishing gated on the default
  branch plus workflow_dispatch
• unsigned for now, so clients need [trusted=yes]; signing and keeping older builds are
  separate decisions

echo "deb [trusted=yes] https://<org>.github.io/sems/debian12 ./" \
  | sudo tee /etc/apt/sources.list.d/sems.list
sudo apt update && sudo apt install sems

Tested: five suites green in one ~9 min run, tree deploys, Ubuntu 22.04 host installs
with plain apt.

[denyspozniak]

Running from my fork if it helps to see it: https://denyspozniak.github.io/sems/ - five
suites, newest three builds each, every directory browsable.

echo "deb [trusted=yes] https://denyspozniak.github.io/sems/debian12 ./" \
  | sudo tee /etc/apt/sources.list.d/sems.list
sudo apt update && sudo apt install sems

rpm_test_build is red on master too (run 28855397667, same step); the publishing jobs
skip on pull_request by design, so a PR never deploys.