rustPackages: make rustc-dev optional

This was only introduced in 1.40.0 and doesn't work on older versions.

    thread 'main' panicked at 'Error: no rules matched rustc-dev.', src/bootstrap/builder.rs:231:21
    note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.
    failed to run: /build/rustc-1.38.0-src/build/bootstrap/debug/bootstrap dist rustc-dev