winrtrunner exits with error "The environment variable ExtensionSdkDir is not set."

winrtrunner exits with error "The environment variable ExtensionSdkDir is not set."

To reproduce, just uninstall VCLibs from your system (Remove-AppxPackage in PowerShell). You can see the effect of the build environment on winrtrunner by unsetting ExtensionSdkDir in Creator.