Use the Rust release version in build info

[ryanfowler]

Summary

• Store the compiler version as the plain Rust release number in FETCH_RUSTC_VERSION
• Keep --buildinfo output compact by showing values like 1.95.0 instead of the full rustc ... banner
• Update the build-info test to assert the rust field is whitespace-free and no longer includes rustc

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• Focused unit test for build-info JSON passed