Add beta/stable integration testing

[brson]

Right now our beta and stable branches have no automatic integration testing set up for them. Requires changes to homu and rust-buildbot, as well as decisions about what the workflow is.

Originally part of #16677. Won't be finished for 1.0 beta.

[brson]

Removing from 1.0 and unassigning myself. We have a manual process that is working fine for now.

[brson]

I'm happy with how the process works right now. Closing.