/etc/profile: try all nix profiles for ASPELL_CONF

Aspell can only handle one dict-dir directive and currently we hardocde
that to
  ASPELL_CONF="dict-dir $HOME/.nix-profile/lib/aspell"

This means that aspell doesn't work if it is installed to the system or
default nix profile -- it only works in the user profile.

With this change, aspell can be installed to any of the nix profiles. If
it is installed in more than one profile, the most "local" profile wins
(i.e. sysadmin can set up a default, users can override it).