haskell: fix x509-system on mojave

darwin.security_tool is currently broken in Mojave. See issue #45042
for more info. Our security_tool stuff comes from 10.9 so I suspect
that it needs an update.

Here I am putting in a hack to get things working again. This uses the
system provided security binary at /usr/bin/security to avoid the
issue in Haskell’s x509-system package. Unfortunately, this will break
with the sandbox. I am also working on a proper fix, but this requires
updating lots of Apple stuff (and also copumpkin’s new CF). You can
follow the progress on this branch:

  https://github.com/matthewbauer/nixpkgs/tree/xcode-security

This commit should be backported to release-18.03 and release-18.09.

/cc @copumpkin @lnl7 @pikajude