curl-ca-bundle: better instructions for cert.pem

When using homebrew's openssl, setting the environment variable
`SSL_CERT_FILE` only works for things that use the user's shell.

A better solution that works for all users and shells is to link
`ca-bundle.crt` to `/usr/local/etc/openssl/cert.pem`

Another solution might be to fetch the `ca-bundle.crt` when
building openssl if the `cert.pem` doesn't already exist.

It's really annoying that this "Just Work" out of the box.