Auto merge of #117771 - tmandry:fuchsia-gha, r=Mark-Simulacrum

Build Fuchsia in CI

Fittingly, when I first put this up it was failing due to discovering an ICE in clippy (looks like fixed in rust-lang#11760), probably more fallout from recent type system changes. Other recent regressions this would have caught include

- #117455 and #117493
- #117602

Originally we discussed basing this on cargotest, but they ended up not sharing anything. Fuchsia has its own tool to manage checkouts and its own build system. What it requires is a fully "install"ed toolchain with a host and fuchsia target. We share logic from the dist-various-2 builder to build the fuchsia target.

Right now this runs clippy and skips linking a bunch of targets, since most issues we catch are in the frontend. In theory we could probably get the build CPU time down quite a bit with this approach, but right now some linked targets are creeping into the dependencies anyway and we don't have a good way of preventing that yet.

The approach is basically to get a checkout at a pinned commit and then run a [script](https://fuchsia-review.git.corp.google.com/c/fuchsia/+/943833/6/scripts/rust/build_fuchsia_from_rust_ci.sh) at a predetermined location. I would like to update that pin every few weeks. Partial checkouts are used to minimize clone time, but we don't filter out prebuilt packages.

r? `@Mark-Simulacrum`

Based on discussion in [this Zulip thread](https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/Putting.20Fuchsia.20in.20crater).

Author: bors