We move Clippy lints into separate workflows that only run either
directly on branches, or on trial-merges of PRs, to deduplicate the
lint annotations. We will still see stable lints duplicated in beta,
but that's fine.
We don't want the noise of changes to nightly clippy, but we _do_ want
advance warning of upcoming CI breakage. Running clippy on beta will
give us six weeks notice.
We don't need to commit to an MSRV yet, and this avoids the problem
where some of our in-flux dependencies bump their MSRV (or don't have
a defined MSRV and make changes to use more recent features) causing
our CI to break.