curl: CURLOPT_PROXY_CAINFO_BLOB silently activates native CA store on Apple builds

Hi all, CURLOPTPROXYCAINFOBLOB introduced 7.77.0 never sets proxyssl.customcablob. On USEAPPLESECTRUST / CURLCANATIVE builds this causes curl to silently fall back to the system keychain for proxy TLS verification, nullifying the caller's blob-only trust policy. --- Root cause lib/setopt.c handle...