fix(cli): use shell on win32 for `update` (#1503) (#1504)

If there is no `npm.exe` on the system, but
instead an `npm.cmd`, then node won't find
the `npm` executable when calling `spawnSync`.

This occurs frequently when using node package
managers on Windows.

See the node documentation for `.bat` and `.cmd`
files here.

<https://nodejs.org/api/child_process.html#spawning-bat-and-cmd-files-on-windows>.