As discussed in https://bugs.launchpad.net/ubuntu/+source/snapd/+bug/1640514
shells such as zsh do not default to processing /etc/profile.d on
session setup. PR #5390 used the /etc/environment.d mechansim to make
sure $PATH was updated in these cases. However we also need to do the
same for $XDG_DATA_DIRS in order for snap .desktop files to be visible
to the desktop session.
The default value for XDG_DATA_DIRS comes from the XDG Base Directory
specification.
This addresses LP: 1771858 and ensures that /snap/bin is available in
the PATH everywhere. This superseeds #5226 which was using a full
generator but we really just need the simpler environment conf file.
