Add a cron-scheduled CI runner that runs all tests (nightly build)

[lefou]

Mills test suite grows and we have a hand-picked selection which we always run. But there are tests that never run automatically and some that we probably don't run by accident.

The idea is, to add a simple CI job without tight timeouts, that runs on a fixed interval, e.g. weekly, and runs all available tests at least once.

[lefou]

What we probably should include:

• mill __.compile
• mill __.packaged.daemon.testCached
• mill __:^IntegrationTestModule.__.testCached