creating a branch for PR builds

Author: nvarscar

build/build_prep.ps1

@@ -1,4 +1,8 @@
 dotnet new tool-manifest
 dotnet tool install Cake.Tool --version 1.1.0
 Invoke-WebRequest https://cakebuild.net/download/bootstrapper/dotnet-tool/windows -OutFile build.ps1
-git fetch --all
+if ($prNo = $env:APPVEYOR_PULL_REQUEST_NUMBER) {
+    Write-Host "PR Build, creating an explicit branch"
+    git fetch --all
+    git checkout -b "pr-build-$prNo"
+}