Move the PowerShell code from .github/workflow/cmake-builds.yml into its
own script, cmake/github_v141_xp.ps1, to keep cmake-buils.yml readable.
The script also facilitates synchronizng with the Chocolatey installer's
Wait-VSIstallerProcesses function easier (from which this this code is
derived.)
cmake-builds.yml: Check the output from Get-VSSetupInstance and
Set-VSSetupInstance. Empty (or null) output indicates that the v141_xp
install did not complete successfully. Build process will bail when that
happens.
cmake/v141_xp_install.ps1: Unused and now unnecessary script.
