Patch CLI ShouldGenerateAspNetCertificate to accurately tell user if a cert was generated

[dagood]

#418 (review):

[@peterhuene] I think we should also patch this:

        private bool ShouldGenerateAspNetCertificate()
        {
#if DOTNET_BUILD_FROM_SOURCE
            return false;
#else
            var generateAspNetCertificate =
                _environmentProvider.GetEnvironmentVariableAsBool("DOTNET_GENERATE_ASPNET_CERTIFICATE", true);

            return ShouldRunFirstRunExperience() &&
                generateAspNetCertificate &&
                !_aspNetCertificateSentinel.Exists();
#endif
        }

to prevent the first run experience from telling users a certificate was created when one wasn't.