Bump Fuchsia, build tests, and use 8 core bots - Build Fuchsia on 8 cores instead of 16 - Skip building cranelift for Fuchsia - Bump Fuchsia (includes building tests) This includes a change to the upstream build_fuchsia_from_rust_ci script that builds a minimal set of tests, to improve coverage on this builder. This would have caught https://github.com/rust-lang/rust-clippy/issues/11952 and #119593. See prior discussion on #119400 about building on 8 cores instead of 16. This PR combines changes from that and #119399, plus clean up. r? `@Mark-Simulacrum`