Auto merge of #120358 - tmandry:bump-fuchsia-8c-tests, r=Mark-Simulacrum

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 rust-lang/rust-clippy#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`

Author: bors