CI: Inefficent caching of rust builds

I've looked at a few of the latest (successful) runs of the ci.yml workflow and the swatinem/rust-cache steps show that no cache was found. Additionally, clippy jobs are not cached at all.

I think the that actions is not very good at restoring from branches that are close to a cache hit and we can do better with a custom action; in this repo's .github/actions directory or in a different repo in this github organization.