use Cirrus CI

- 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.

pkgs/default.nix

@@ -24,8 +24,4 @@ let self = {
   lib = import ./lib.nix { inherit (pkgs) lib; };
 
   modulesPkgs = self // self.pinned;
-
-  # Used in ../.travis.yml
-  clightning-plugins-all = pkgs.writeText "clightning-plugins"
-    (pkgs.lib.concatMapStringsSep "\n" toString (builtins.attrValues self.clightning-plugins));
 }; in self