ci: drop nightly test checks, add bisect-ability

[nyonson]

Testing the waters with this one.

1. Add the --baseline arg to the test command so tests are run on every commit of a PR, enforcing bisect-ability. The funky {{ github.event.pull_request.base.sha || github.event.before }} arg is a github-ism so that the command works both pre-merge and post-merge into master.
2. Drop the nightly compiler from the test matrix. I might be missing something, but I don't think the nightly tests are very valuable. We are running lint on nightly, so there is still a high chance we will hear about a breaking change early on. But let me know if this is a bad assumption.