fix: compilation with `default-features = false`

[DaniPopes]

Fix compilation with default-features = false, meaning without the k256 feature, also testing it in CI.

I took the opportunity to also improve the CI with more checks, I adapted it from what we have at alloy-rs/core.

[DaniPopes]

Yes, these are intentional.

• Binaries are also not compiled every time, but downloaded from GH releases, see taiki-e/install-action
• I don't think having multiple action repos matters
• cargo test is perfectly fine on a small repo with only unit tests; same for ungrouped actions, I'd prefer seeing what exactly failed (clippy vs fmt vs build) rather than having everything in one job

Running time went from 3m cached to 40s uncached (https://github.com/sigp/enr/actions/runs/8618027272). It technically does use more resources and has more billable time but running it is free so it ultimately doesn't matter.