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.

ci/build.nix

@@ -0,0 +1,14 @@
+let
+  pkgs = import <nixpkgs> {};
+  nbPkgs = import ../pkgs { inherit pkgs; };
+  ciPkgs = with nbPkgs; [
+    electrs
+    elementsd
+    hwi
+    joinmarket
+    lightning-charge
+    lightning-loop
+    nanopos
+  ];
+in
+pkgs.writeText "ci-pkgs" (pkgs.lib.concatMapStringsSep "\n" toString ciPkgs)