ci: fix rustfmt check env spelling

[Luzijano]

Summary

• fixes the CI matrix environment variable from RUSTFMTCHK to RUSTFMTCHECK
• makes the stable Rust CI job actually enter the existing rustfmt check branch in contrib/test.sh
• keeps the change limited to CI wiring; no source behavior changes

Validation

• RED/proof before fix: a repository-local assertion failed because .github/workflows/rust.yml exported RUSTFMTCHK while contrib/test.sh reads RUSTFMTCHECK, meaning rustfmt was skipped
• GREEN/proof after fix: the same assertion passes and confirms the workflow/script spelling matches
• bash -n contrib/test.sh
• git diff --check

Note: cargo fmt --all -- --check could not be run in this cron environment because rustup/the cargo fmt component is not installed here; this PR enables the repository's existing CI to run that check on GitHub.

AI/tooling disclosure: prepared with Hermes Agent automation.