build.nix: Add option to build with assertions.
build.nix: Add option to build with assertions.

tempbottle/badvpn,linfengfeiye/badvpn,Git-Host/badvpn,linfengfeiye/badvpn,binondord/badvpn,LazyZhu/badvpn,LazyZhu/badvpn,binondord/badvpn,binondord/badvpn,LazyZhu/badvpn,PowerOlive/badvpn,PowerOlive/badvpn,PowerOlive/badvpn,LazyZhu/badvpn,tempbottle/badvpn,PowerOlive/badvpn,Git-Host/badvpn,binondord/badvpn,linfengfeiye/badvpn,Git-Host/badvpn,PowerOlive/badvpn,binondord/badvpn,tempbottle/badvpn,tempbottle/badvpn,Git-Host/badvpn,linfengfeiye/badvpn,tempbottle/badvpn,Git-Host/badvpn,LazyZhu/badvpn,linfengfeiye/badvpn