Missing env variable NUM_JOBS for cargo build script

[jgao54]

While using rules_rust with cargo-raze, I encountered an issue where the build script for a dependency crate jemalloc-sys 0.3.2 expects env var NUM_JOBS, which cargo sets by default for build scripts.

Not sure if NUM_JOBS is intentionally left out from cargo_build_script_runner/bin.rs or not -- happy to create a PR to add it.

There's also a related issue tracked in cargo-raze to support passing in custom build env vars. However, given this is a default cargo environment variable this might be a better place to add it.

[illicitonion]

Good catch, thanks for the bug report! Fixed in #1216 :)