Fix panic & use `cmd` to run `yarn`&`npm` on windows

Editor assessment

The subject states both the crash and the remedy: panics are fixed and yarn and npm are now spawned through cmd on Windows. The ampersand punctuation is cramped, but the platform-specific mechanism is concrete and recognizable to anyone who has hit Windows process-spawning issues. Type fix and scope cli.rs are accurate. No body, acceptable since the failure and the workaround are both stated. No footers beyond the PR number. A complete, practical fix message.