ddclient: Set SSL_CERT_FILE environment variable

Otherwise connection to SSL hosts fails like this:

May 26 06:44:05 kbuilder ddclient[17084]: WARNING:  cannot connect to dynamicdns.park-your-domain.com:443 socket:
    IO::Socket::IP configuration failed SSL connect attempt failed with unknown error
    error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed