Use no substituers by default in the tests
Use no substituers by default in the tests

Otherwise https://cache.nixos.org is chosen by default, causing the OSX
testsuite to hang inside the sandbox.

(In a way, this is probably rugging an actual bug under the carpet as
Nix should be able to gracefully timeout in such a case, but that's
beyond mac OSX-fu)

NixOS/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,NixOS/nix,NixOS/nix,rycee/nix,rycee/nix