Export `MAX_JOBS` or do whatever is necessary to support jobserver while running build scripts

[nagisa]

The cc crate, very commonly used in build.rs scripts supports parallel compilation, however it appears that within nix it will only end up using 1 job regardless of any other factor.

As per https://github.com/alexcrichton/cc-rs/blob/master/src/lib.rs#L32 we should be setting MAX_JOBS or perhaps even look into supporting jobserver?