When users run setup.ps1 from PowerShell with cwd outside the repo
(e.g. C:\Users\Admin>), npm 11 walks up looking for package.json and
errors with ENOENT before honoring --prefix. setup.bat already pushd's
into core; setup.ps1 was missing the equivalent Push-Location guard.
Fixes#27 (setup.ps1 ENOENT package.json reported by lawler-code).
88cf2b2dc4