Fix Nix Cargo hash and Rust toolchain pin

[jessfraz]

Refresh the Cargo vendor hash for the current lockfile and update rust-overlay so Nix can resolve Rust 1.99.1.

Rebuilt the vendor-staging derivation and checked its hash. nix flake check --all-systems --no-build and the Nix formatting check pass. Full package builds are left to CI.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.90%. Comparing base (d6bad2e) to head (0ad1f17).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1908   +/-   ##
=======================================
  Coverage   67.90%   67.90%           
=======================================
  Files          43       43           
  Lines        8705     8705           
=======================================
  Hits         5911     5911           
  Misses       2794     2794           

Flag	Coverage Δ
unittests	67.90% <ø> (ø)

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