libfetch: use default CA path rather than default CA file.
libfetch: use default CA path rather than default CA file.

The default CA file set by FreeBSD is only available when using the
LibreSSL provided CA file, and we've decided to use the CA path
by default.

Discussed with @dominikh.

stpx/xbps,stpx/xbps,ebfe/xbps,ebfe/xbps,stpx/xbps,ebfe/xbps