Function prepare_rustc

fn prepare_rustc(
    build_runner: &BuildRunner<'_, '_>,
    unit: &Unit,
) -> CargoResult<ProcessBuilder>

Prepares flags and environments we can compute for a rustc invocation before the job queue starts compiling any unit.

This builds a static view of the invocation. Flags depending on the completion of other units will be added later in runtime, such as flags from build scripts.