win,msi: install Boxstarter from elevated shell

Boxstarter asks for elevation to install packages, but not to install
Boxstarter itself. Thus, run all the commands from an elevated
PowerShell.

Refs: https://github.com/nodejs/node/pull/22645
PR-URL: https://github.com/nodejs/node/pull/22988


Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>