There are three files at least, that control UBT’s use of cores.
The one I use so that every built of UE4, the launcher versions and the github versions, use all cores is:
C:/Users/[your user name here]/AppData/Roaming/Unreal Engine/UnrealBuildTool/BuildConfiguration.xml
…with contents:
<?xml version="1.0" encoding="utf-8"?>
<Configuration xmlns="https://www.unrealengine.com/BuildConfiguration">
<BuildConfiguration>
<ProcessorCountMultiplier>2</ProcessorCountMultiplier>
</BuildConfiguration>
</Configuration>