Temp dirs not cleaned up on Windows

[ethanndickson]

We currently don't clean up temp directories in ~/.mux-tmp on Windows, as the pwd command returns a POSIX-path, which is then passed to a Node API to clean up the directory. This means whenever a stream ends a log is produced like:

Failed to cleanup temp dir /c/Users/ethan/.mux-tmp/854e934d: RuntimeError: Working directory does not exist
  path: 'C:\c\Users\ethan\.mux-tmp'

See #968 for a failed attempt at fixing.