Needed by the following commits.
Also, don't use the cachix cache as a substituter for local, non-CI
builds. This obviates the need for the 'untrusted' warning in build.sh.
- 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.
