travis: cache all build outputs with cachix

This further speeds up builds, in particular the modules test in the
next commit. By checking if the expected final build output has already
been cached, we can even skip the download of cached builds.

helper/wait-for-network-idle.rb

@@ -0,0 +1,29 @@
+#!/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)