ci: need a CI for the bindings

[SteVwonder]

It would be good to get a nightly CI for the rust bindings so that we are alerted when they break based on changes to flux-core.

I've already created a Dockerfile based on fluxrm/flux-core:focal that could be used. I think the next step is setting up some Github Actions based on that image (since Travis is limiting the number of CI cycles and the Github cron notification is much better IMO).