nixos: add 'gvfs' when using GNOME3 desktop

One reason for adding this is to make Chromium able to open files it has
downloaded.

Currently this happens:
  /run/current-system/sw/bin/xdg-open: line 364: gnome-open: command not found

(And nothing happens in the GUI when clicking a downloaded file.)

Looking into xdg-open, one can see that it first tries to run gvfs-open
and then falls back to gnome-open. Adding 'gvfs' makes the first command
succeed.
Merge pull request #2744 from bjornfor/gnome-desktop-needs-gvfs

nixos: add 'gvfs' when using GNOME3 desktop