CI: have nightly job to catch unstable feature changes and regressions #406
Description
From #402 (comment):
Can you keep at least a single nightly job as canary in case rustc changes unstable features or there is a regression?
We had this in the past where we ran several nightly versions, but now that we moved to the beta/stables, we lost that.
Note that we will need to disable it if something breaks, since we will be pinning the version to stable releases, but it is good to spot when that happens and what exactly have changed. Thus, ideally, we will have a separate periodic job/report to check for this, instead of doing it in the CI for the PRs.