Fix runInMachine

It requires a writable /nix/store to store the build result.  Also,
wait until we've reached multi-user.target before doing the build, and
do a sync at the end to ensure all data to $out is properly written.

http://hydra.nixos.org/build/6496716
Disable the trac test

It's been failing since forever.
xorg-server: Fix a 20 year old security bug

CVE-2013-4396