colord: fix use of /var

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.