CI Cache Enhancements

[posborne]

CI had failed in the most recent set of changes which touched Cargo.lock (invalidating cache). Things were timing out in clippy. This change gets CI passing again by implementing a few improvements:

• Avoid running CI on branch push (can still trigger manually in GH)
• Caching of sources was separated from binary caches.
• Sccache was using a deprecated node version and not giving us anything, removed.
• For clippy, we match the flags from our maturin compile stage which let's us avoid another rebuild. This makes sense for now; if it stops making sense then we could move clippy to do its build in parallel with everything else and that might solve our problem in another way (though what we cache becomes messier, so this seems preferable).
• Include the RUSTUP_TOOLCHAIN version in the cache key. If our toolchain version changes, we need new binaries and this helps avoid bloat (source isn't impacted by this).