extend `detect_src_and_out` test
> I was thinking about the following cases when I wrote the comment in #109055
>
> 1. Running bootstrap from the source root.
> 2. Running from a subdirectory of the source root.
> 3. Running from outside the source root.
> 4. Running on a different machine from where bootstrap was compiled (which will be important > for #107812). You can mostly replicate this by renaming the source root so it no longer exists on disk.
> 5. Running with `--build-dir`.
> 6. Running with `$RUST_BOOTSTRAP_CONFIG` set in the environment and `build-dir` set in the file.
Tested all the topics mentioned above. All worked fine. The test is now also covers if build dir is manually specified in config.
r? `@jyn514`
helps #109120 partially
See https://github.com/rust-lang/compiler-team/issues/566.
The motivation for changing the default is to avoid downloading and building LLVM when someone runs `x build` before running `x setup`.
The motivation for only doing it on `channel = "dev"` is to avoid breaking distros or users installing from source. It works because `dev` is also the default channel.
The diff looks larger than it is; most of it is moving the `llvm` branch below the `rust` so `config.channel` is set.
