You shouldn’t need to reinstall any dotnet core because the 5.0EA already ships with the correct dotnet core. The automation scripts just don’t seem to use it. You can work around this by setting the environment variable DOTNET_ROOT to %ENGINE_DIR%\Engine\Binaries\ThirdParty\DotNet\Windows (replace %ENGINE_DIR% with your 5.0EA install directory).
This is applicable if you see the hostfxr.dll error or if you see something like “The specified framework ‘Microsoft.WindowsDesktop.App’, version ‘3.1.0’ was not found”.
In my opinion this could be a simple fix on Epic’s side to actually use the dotnet core that the engine comes with.