Use sparse registry for Cargo

[giordano]

When it will become available in a stable version of Rust we support, it'd be nice to automatically enable the sparse registry: https://blog.rust-lang.org/2022/06/22/sparse-registry-testing.html. Updating the registry always takes an overly large fraction of the build process of Rust packages, in addition to the fact we can't cache build outputs (related issue: #186).

[giordano]

For the record, rust-lang/cargo#11224 was merged and should make it to the next version of Rust.

I believe that on our side we only need to set the environment variable

CARGO_REGISTRIES_CRATES_IO_PROTOCOL=sparse

Update:

Sparse registries are now stabilized and will be available in Rust 1.68, which will be released on 2023-03-09