update build.ps1 to put downloaded dotnet in PATH

build.ps1

@@ -62,6 +62,8 @@ else {
     } else {
         ExecSafe { & $DotNetInstallFile -InstallDir $DotNetDirectory -Version $DotNetVersion -NoPath }
     }
+
+    $env:PATH="$DotNetDirectory;$env:PATH"
 }
 
 Write-Output "Microsoft (R) .NET Core SDK version $(& $env:DOTNET_EXE --version)"