Pass -prerelease to the vswhere query

On our 15.8 machines, there aren't release versions...because we haven't
released it yet.

build/scripts/build-utils.ps1

@@ -353,7 +353,7 @@ function Test-SupportedVisualStudioVersion([string]$version) {
 # meets our minimal requirements for the Roslyn repo.
 function Get-VisualStudioDirAndId() {
     $vswhere = Join-Path (Ensure-BasicTool "vswhere") "tools\vswhere.exe"
-    $output = Exec-Command $vswhere "-requires Microsoft.Component.MSBuild -format json" | Out-String
+    $output = Exec-Command $vswhere "-prerelease -requires Microsoft.Component.MSBuild -format json" | Out-String
     $j = ConvertFrom-Json $output
     foreach ($obj in $j) {