NetworkPkg/TlsAuthConfigDxe: preserve TlsCaCertificate variable attributes

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.

Cc: Jiaxin Wu <jiaxin.wu@intel.com>
Cc: Siyuan Fu <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Jiaxin Wu <jiaxin.wu@intel.com>