Non-named PowerShell installer broken

[MarkTiedemann]

iex (iwr https://deno.land/x/install/install.ps1)

iex doesn't work with param. See https://ci.appveyor.com/project/MarkTiedemann/win-ci-playground/builds/21844216.

So apparently this installation method hasn't worked since #22.

This is easy to fix, luckily:

if ($args.Count -gt 0) {
  $Version = $args.Get(0)
}

I already tested it, and it works: https://ci.appveyor.com/project/MarkTiedemann/win-ci-playground/builds/21844766.

Will make a PR for it tomorrow and also add proper end-to-end tests for that.