[ci] freshen up

[sunshowers]

• Use ubuntu-22.04, and macos-14 for M1 runners.
• Move clippy configuration to Cargo's config rather than lib.rs or CI.
• Copy over omicron's clippy config.
• Set CARGO_TERM_COLOR=always for easier to read GHA output.
• Cache artifacts for quicker builds.
• Update README.

Overall, it's a nice speedup. There's still more potential for improvement via things like hakari which are more invasive.

• On Ubuntu, compared to this baseline which took 4m3s, this build reused the cache and took 2m43s, around 1.5x as fast.
• On macOS, compared to this baseline which took 6m15s, this build reused the cache and took just 2m5s. This is 3x as fast.