The latest nixpkgs-unstable update would require node-based packages to
distinguish between stable and unstable when building. Instead of dealing with
that complexity, we will only guarantee compatability of our packages with
stable.
- Don't fail on `kill $cachixPid` when cachix has already exited.
This fixes some failing CI builds.
- Stop the cachix background worker before the final `cachix push`.
This can avoid unneeded reuploads.
Use the coreutils version of tail on cirrus which supports
argument --pid.
- Make more economic use of the free CI resources by removing redundant build tasks:
- Build unstable pkgs in a single separate task ("pkgs_unstable").
- All stable pkgs are implicitly built by the modules tests.
- The build script (ci/build.sh) can now be executed locally for easier
debugging.
- Use an explicit 'cachix push' command instead of helper/wait-for-network-idle.rb.
This is simpler and more reliable.
