Run periodic CI against Tor @ head

[sporksmith]

We want to know when something in Tor changes that breaks compatibility with Shadow. Eventually we'd like such testing to happen in Tor's CI, such that changes to Tor that break compatibility with shadow-plugin-tor are blocked. Before that happens we need this code to be more stable, and ideally migrate some of it into Tor itself.

In the meantime we'll set up a periodic (~daily) workflow in this repository that tests tor@head with shadow-plugin-tor@stable.