Generic CI improvements

[danobi]

From: #40 (review)

Couple of nice-to-have things:

• That's highly controversial opinion, but I personally prefer cargo fmt to be triggered by cargo test, rather than by my CI, so that I can run cargo test local and not worry about some checks which are there on the CI but are missing locally. The same applies to static linking --- if I am worried about being able to statically link, I'd probably write an integration tests for this which shells out to cargo, rather than leaving this logic to a .yml in CI.
• default caching on CI is pretty bad, https://github.com/Swatinem/rust-cache/actions is a big improvement if you care about CI times.