ILProj + Signing with open key is broken

ILAsm doesn't support full signing. It doesn't look like it will in the near future: https://github.com/dotnet/coreclr/issues/22862#issuecomment-467591230

To deal with this Arcade should have a copy of the open.snk that only contains the public key and define a combination of property values that can be used in the ilproj case so that we delay-sign.