workaround dotnet build order issue

Rx.NET/Source/build-new.ps1

@@ -28,6 +28,11 @@ Write-Host "Version: $version"
 Write-Host "Restoring packages" -Foreground Green
 dotnet restore $scriptPath | out-null
 
+#need to ensure core is built first due to bad dependency order determination by dotnet build
+
+$coreDir = gci . -Directory | where-object {$_.Name -eq "System.Reactive.Core"} | Select -First 1
+dotnet build -c "$configuration" $coreDir.FullName
+
 Write-Host "Building projects" -Foreground Green
 $projects = gci $scriptPath -Directory `
    | Where-Object { ($_.Name -notlike "*DeviceRunner") -and (Test-Path (Join-Path $_.FullName "project.json"))  } `