Failure on Windows when running `npm ...`

[stefnotch]

The following script works perfectly fine on Linux systems, but will fail on Windows (command not found)

cmd!(sh,"npm --help").run()?;

The reason for that seems to be related to
rust-lang/rust#42791

I'm not sure if there's an easy fix for this at all, but I figured that it would be worthwhile to at least open an issue for it. This way, it's at least documented somewhere.