(#8013) [vcpkg] Compatibility with pwsh >= 6.2

In Powershell 6.2/7.0, `Get-ChildItem`'s paths are now printed as
absolute paths instead of relative paths; this breaks the code we have
that assumes that they're printed as relative paths.

Thus, change the code to request the `Name` property specifically, which
is what we actually wanted.

scripts/bootstrap.ps1

@@ -226,8 +226,9 @@ function getWindowsSDK( [Parameter(Mandatory=$False)][switch]$DisableWin10SDK =
         $win10sdkVersions = @(Get-ChildItem $folder | Where-Object {$_.Name -match "^10"} | Sort-Object)
         [array]::Reverse($win10sdkVersions) # Newest SDK first
 
-        foreach ($win10sdkV in $win10sdkVersions)
+        foreach ($win10sdk in $win10sdkVersions)
         {
+            $win10sdkV = $win10sdk.Name
             $windowsheader = "$folder\$win10sdkV\um\windows.h"
             if (!(Test-Path $windowsheader))
             {