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