I was unable to find a reliable fix for the problems.
I seem to get either this http://hydra.nixos.org/build/5326327/nixlog/1/tail-reload
or that http://hydra.nixos.org/build/5297462/nixlog/1/tail-reload .
# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch x-updates
# You are currently rebasing branch 'x-updates' on '81b7f5f'.
# (all conflicts fixed: run "git rebase --continue")
#
# Changes to be committed:
# (use "git reset HEAD <file>..." to unstage)
#
# modified: pkgs/development/libraries/qt-4.x/4.8/default.nix
#
# Untracked files:
# (use "git add <file>..." to include in what will be committed)
#
# core
# dev.diff
# pkgs/applications/networking/browsers/firefox/cairo-test.patch
# pkgs/applications/video/flowblade/
# pkgs/desktops/gnome-3/core/dconf/
# pkgs/desktops/gnome-3/core/eog/
# pkgs/desktops/gnome-3/core/libsecret/
# pkgs/development/libraries/ncurses/builder.sh
# pkgs/servers/x11/xorg/download-cache/
# pkgs/servers/x11/xorg/still-missing.txt
cba6d9b11a