Currently cargo-setup-hook instructs the builder upon cargoSha256 or
cargoHash being out-of-date compared to the Cargo.lock file.
The instructions can be simplified a bit, because nowadays it is fine to
keep a hash empty, instead of filling it with
`0000000000000000000000000000000000000000000000000000`.
Nix nowadays outputs SRI hashes, which should usually be placed in
`cargoHash` instead of `cargoSha256`, but the instructions are still
only referring to `cargoSha256`.
Lastly, the output of Nix doesn't include `got: sha256: ` anymore, as it
now outputs `got: sha256-XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX=`.
It would be nice to make it clear that the trailing `=` is important as
well, so the full example SRI hash is mentioned.
e3a10a12c7