Add more CI coverage

[danobi]

Various small tweaks. See individual commits for more details.

[danobi]

cc @matklad

[matklad]

LGTM, but couple of points about overall CI:

• I usually cargo test --no-run rather than cargo build, to compile the tests as well
• I think actions/checkout is at v3 these days? Not that it matters
• If we want to check static linking, we might build against -musl rather than -gnu
• 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.
• I like adding -Dwarnings to RUSTFLAGS on CI --- I don't mind warnings locally when I develop, but I'd rather CI not allowed me to merge them to master
• default caching on CI is pretty bad, https://github.com/Swatinem/rust-cache/actions is a big improvement if you care about CI times.

[danobi]

@matklad thanks for the tips! I added most of those suggestions to this pr. I filed a ticket to track the other two cuz I have various thoughts on that as well.

Nesting cargo calls for rustfmt is a neat trick -- I'll have to remember that. But for static build, it kinda requires a bit more setup like having musl-gcc wrapper script installed. I've done some similar "advanced" stuff for CI before and I found a nix based approach to be really awesome: https://dxuuu.xyz/bpftrace-nix.html