/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).
release.nix: Automatically include all of tests/default.nix
Test whether the transparent fetchurl cache (tarballs.nixos.org) works

Testing this is useful in any case, but it's necessary now because Nix
1.6 doesn't check the binary cache for fetchurl output anymore.

http://hydra.nixos.org/build/6144188
nixos-install: Fix copying from the CD

Nix 1.6 doesn't run the copy-from-other-stores substituter by default
anymore, so turn it on explicitly.

http://hydra.nixos.org/build/6144173
try to fix zfs into initrd