Add `[patch.crates-io]` to unify duplicate registry/git crate sources

[aldoborrero]

The workspace Cargo.lock contains three crates that appear from both crates.io and git forks with the same name-version:

• bls12_381 v0.8.0 — registry + https://github.com/lambdaclass/bls12_381 (branch expose-fp-struct)
• halo2curves-axiom v0.7.2 — registry + https://github.com/axiom-crypto/halo2curves.git (tag v0.7.2)
• zkhash v0.2.0 — registry + https://github.com/HorizenLabs/poseidon2.git (rev bb476b9)

This works fine with cargo build (which resolves from the lockfile), but breaks vendoring tools like cargo vendor and Nix's importCargoLock/fetchCargoVendor, which cannot handle two sources for the same package name-version:

error: found duplicate version of package `bls12_381 v0.8.0` vendored from two sources

Adding these lines to the workspace Cargo.toml would unify the sources and fix vendoring:

[patch.crates-io]
bls12_381 = { git = "https://github.com/lambdaclass/bls12_381", branch = "expose-fp-struct" }
halo2curves-axiom = { git = "https://github.com/axiom-crypto/halo2curves.git", tag = "v0.7.2" }
zkhash = { git = "https://github.com/HorizenLabs/poseidon2.git", rev = "bb476b9" }

This tells Cargo to always use the git fork instead of the registry version, eliminating the duplicate. The existing Cargo.lock resolution stays the same — it just makes vendoring work.

Found while packaging ethrex for nix-community/ethereum.nix.