bootstrap/miri: avoid rebuilds for test builds When building Miri in its own repo, we always build with `--all-targets`: https://github.com/rust-lang/rust/blob/a00961269107703772c4e8f071f0accbe0f1a7e5/src/tools/miri/miri-script/src/util.rs#L167-L174 This saves a bunch of time since some of Miri's dependencies get more features enabled by some of Miri's dev-dependencies, and they all get built twice otherwise if you do `cargo build && cargo test` (which is typically what you end up doing inside `./miri test` and also inside `./x test miri`). This applies the same approach to bootstrap, drastically reducing the edit-compile cycle for Miri work here. :)