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.

test/run-tests.sh

@@ -141,7 +141,7 @@ buildTest() {
 # On continuous integration nodes there are few other processes running alongside the
 # test, so use more memory here for maximum performance.
 exprForCI() {
-    memoryMiB=3072
+    memoryMiB=4096
     memTotalKiB=$(awk '/MemTotal/ { print $2 }' /proc/meminfo)
     memAvailableKiB=$(awk '/MemAvailable/ { print $2 }' /proc/meminfo)
     # Round down to nearest multiple of 50 MiB for improved test build caching