This is a squash commit of the joint work from:
* Jan Tojnar (@jtojnar)
* Linus Heckemann (@lheckemann)
* Ryan Mulligan (@ryantm)
* romildo (@romildo)
* Tom Hunger (@teh)
Currently the package is built with /var in $out/var. That fails when it
tries to create/write things at runtime (nix store is read-only).
Instead, tell it to use /var (global directory) and fixup the
installation phase so it doesn't touch /var (leave that for runtime).
This unbreaks the colord dbus service, which apparently is needed by
cups to create color profiles for printers.