Distribute bootstrap in CI This pre-compiles bootstrap from source and adds it to the existing `rust-dev` component. There are two main goals here: 1. Make it faster to build rust from source, both the first time and incrementally 2. Make it easier to add non-python entrypoints, since they can call out to bootstrap directly rather than having to figure out the right flags to pre-compile it. This second part is still in a bit of flux, see the tracking issue below for more information. There are also several changes to make bootstrap able to run on a machine other than the one it was built (particularly around `config.src` and `config.out` detection). I (`@jyn514)` am slightly concerned these will regress unless tested - maybe we should add an automated test that runs bootstrap in a chroot or something? Unclear whether the effort is worth the test coverage. Helps with https://github.com/rust-lang/rust/issues/94829.