chore: Replace build.rs with lint config in Cargo.toml

[aborgna-q]

Rust 1.80 just dropped, adding a lint option that allows us to avoid the build.rs definition added in #386.
(The nightly lint got updated before hitting beta, so there shouldn't be compatibility problems there).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.49%. Comparing base (fe7d40e) to head (b4992a5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #509   +/-   ##
=======================================
  Coverage   83.49%   83.49%           
=======================================
  Files          64       64           
  Lines        6647     6647           
  Branches     6126     6126           
=======================================
  Hits         5550     5550           
  Misses        817      817           
  Partials      280      280           

Flag	Coverage Δ
python	96.73% <ø> (ø)
rust	82.37% <ø> (ø)

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[doug-q]

❤️

This will with older rustcs, we don't care because we only require the latest in CI?

[aborgna-q]

The lint never dropped in stable, so things will still pass there.
The CI error only appeared in the nightly checks, before the lint config was added.