If the platform creates the "TlsCaCertificate" variable as volatile, then
EnrollX509toVariable() shouldn't fail to extend it just because
TLS_AUTH_CONFIG_VAR_BASE_ATTR contains the EFI_VARIABLE_NON_VOLATILE
attribute.
Thus, if the variable exists, add the EFI_VARIABLE_APPEND_WRITE attribute
to the variable's current attributes. This is what DeleteCert() does
already.