[release/8.0] Support using MSBuild from xcopy msbuild tool package though VS installation exists (#15971)

Co-authored-by: Gang Wang <[email protected]>

Author: github-actions[bot]

eng/common/tools.ps1

@@ -417,7 +417,7 @@ function InitializeVisualStudioMSBuild([bool]$install, [object]$vsRequirements =
 
   # Locate Visual Studio installation or download x-copy msbuild.
   $vsInfo = LocateVisualStudio $vsRequirements
-  if ($vsInfo -ne $null) {
+  if ($vsInfo -ne $null -and $env:ForceUseXCopyMSBuild -eq $null) {
     # Ensure vsInstallDir has a trailing slash
     $vsInstallDir = Join-Path $vsInfo.installationPath "\"
     $vsMajorVersion = $vsInfo.installationVersion.Split('.')[0]