Auto merge of #4442 - Xanewok:process-builder-program, r=alexcrichton

Allow to substitute executed program in `ProcessBuilder`

Extends #4424. Sorry for the noise, turns out it'd also be useful to substite the executed program (e.g. for rls, which needs to use a rustc shim itself) without having to copy over manually all parts of the `ProcessBuilder`.

While I'm at it, do you think this needs even more functionality, like being able to specify `None` for the `cwd` or `jobserver` (currently API exposes only ability to specify `Some(...)` as values for those)?

r? @alexcrichton