Add /etc/ssl/certs/ca-certificates.crt symlink for Ubuntu compatibility

NixOS and Fedora uses .../ca-bundle.crt. Ubuntu uses
.../ca-certificates.crt. Add .../ca-certificates.crt symlink to be
compatible with Ubuntu.

Example use case: Bob has a ~/.msmtprc file that he brings over from
Ubuntu. It also works on NixOS.