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.

helper/wait-for-network-idle.rb

@@ -1,29 +0,0 @@
-#!/usr/bin/env ruby
-require 'open3'
-
-# Wait until the given PID had no network activity for `Timeout` seconds, then exit.
-
-pid = ARGV.first
-Timeout = 2
-
-stdin, out, err, wait_thread = Open3.popen3("strace -f -e trace=network -s 1 -q -p #{pid}")
-while IO.select([err], nil, nil, Timeout)
-  begin
-    out = err.read_nonblock(1 << 10)
-  rescue EOFError
-    status = wait_thread.value
-    if status.success?
-      puts "Monitored process #{pid} exited"
-      exit 0
-    else
-      puts "Strace failed with exit code #{status.to_i}. Last output:\n#{out}"
-      # strace often fails with code 256 which looks like success to shells. fail with 1 instead.
-      exit 1
-    end
-  end
-end
-
-# If we exit without an explicit kill,
-# ptrace can fail on reattachment:  ptrace(PTRACE_SEIZE, $PID): Operation not permitted
-# Only relevant for testing.
-Process.kill("TERM", wait_thread.pid)