Dogfood CARGO_REGISTRIES_CRATES_IO_PROTOCOL=sparse in bootstrap

[jyn514]

Sparse-registry was recently stabilized and we've started testing it in CI: #107543

It would be nice to also use it when running locally so that compile times are a bit faster (and to increase coverage). The code to change lives somewhere around

rust/src/bootstrap/builder.rs

Line 1889 in 3905517

cargo.env("RUSTC_INSTALL_BINDIR", self.config.bindir_relative());

We should also undo the changes in #107543 at the same time, since they're now redundant.

[KittyBorgX]

@rustbot claim

[albertlarsan68]

I think there should be a way to opt-out, maybe via an env variable, but it should be discoverable, even though this method will become the default/only.
The benefits will not be that much visible, because the only time it could be needed to update the registry is when first building bootstrap (because the Cargo.{toml, lock} files contain unknown dependencies), or when messing with the dependencies.

[jyn514]

I don't want to add more knobs to bootstrap. If we think this is likely to fail we should just wait until cargo turns it on by default.

That said I don't think it's likely to fail, CI has been using it fine for a few weeks.