flatpak: Use sparse registry protocol

Merged Jamie Murphy requested to merge sparse_protocol into main

Rust 1.68.0 introduces the sparse registry protocol, which significantly reduces the amount of time cargo takes to fetch the crates.io index. This new protocol only downloads the information for crates we need, rather than all crates in the registry.

See https://blog.rust-lang.org/2023/03/09/Rust-1.68.0.html#cargos-sparse-protocol

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