Make BuildAll.ps1 compatible with VS prerelease installs (#3656)

Author: evelynwu-msft

BuildAll.ps1

@@ -68,7 +68,7 @@ if(-not (test-path ".nuget\nuget.exe"))
 $configurationForMrtAndAnyCPU = "Release"
 $MRTSourcesDirectory = "dev\MRTCore"
 
-$VCToolsInstallDir = . "${env:ProgramFiles(x86)}\Microsoft Visual Studio\Installer\vswhere.exe" -Latest -requires Microsoft.Component.MSBuild -property InstallationPath
+$VCToolsInstallDir = . "${env:ProgramFiles(x86)}\Microsoft Visual Studio\Installer\vswhere.exe" -Latest -prerelease -requires Microsoft.Component.MSBuild -property InstallationPath
 write-host "VCToolsInstallDir: $VCToolsInstallDir"
 
 $msBuildPath = "$VCToolsInstallDir\MSBuild\Current\Bin\msbuild.exe"