[build] caching on Ubuntu builders of GH actions does nothing

[tesuji]

See this build log: https://github.com/rust-lang/rust-clippy/pull/6154/checks?check_run_id=1235351421#step:6:1

A solution is to cache only need files and subdirs: https://doc.rust-lang.org/nightly/cargo/guide/cargo-home.html#caching-the-cargo-home-in-ci.
We might also want to cache .crates.toml if we use cargo install.

[flip1995]

Yes, we're waiting for actions-rs/install#5 to get through, so we can do this more cleanly.