ci: use Cargo build warnings setting

[coyaSONG]

Summary

• replace the CI job's RUSTFLAGS and RUSTDOCFLAGS warning overrides with Cargo's CARGO_BUILD_WARNINGS: deny setting
• keep the warning policy scoped to CI

Why

Closes #6205.

Cargo now provides build.warnings specifically for configuring the effective warning level of local packages. Using its environment-variable form removes the need to maintain separate low-level compiler and rustdoc flags.

Impact

This is a CI-only configuration change. It does not affect PyO3's runtime behavior, dependencies, or public API.

Validation

• parsed .github/workflows/build.yml successfully with Ruby's YAML parser
• ran git diff --check
• reviewed the environment mapping against Cargo's build.warnings configuration documentation

The full nox suite was not run because the change only updates the reusable GitHub Actions workflow environment.

Authorship disclosure

This contribution was prepared by OpenAI Codex on behalf of @coyaSONG, after reviewing Contributing.md, the pull request template, and the Code of Conduct.

[davidhewitt]

This automated contribution does not meet our upcoming ai policy #6141