Revise workflows in light of advances in the state-of-the-art

[mpalmer]

• Move actions/checkout and actions/cache to @v3, to shut up node runtime version warnings;

• Get rid of unmaintained actions-rs actions, mostly replace with straight cargo run commands;

• Add a new nightly_compat audit job, that'll build against Rust nightly (and latest deps) and fail if anything breaks;

• Add some more caching;

• Prevent CI from failing if nodejs tests fail (because we know they will at the moment, because it's experimental);

• Use a non-ancient version of action-validator to self-check workflows.